favorable effect upon all the functions which, take part in the changes through which the food must pass before being converted into tissue, namely, digestion, absorption, circulation, oxidation, and assimilation; but it likewise aids in preventing derangement of these functions—that is, a large number of diseases. The following résumé 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 excretion of the products of tissue waste is enhanced. These matters 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 kidneys. 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, drowsiness, 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 substances, therefore, many slight ailments, which, however, are sufficient 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, therefore, 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 increased 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 oxygen 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 respiration increases, a larger quantity of oxygen enters the system;