The rate of breathing is accelerated whenever the body is engaged in muscular exertion, and with this quickened breathing there is an increased amount of oxygen drawn in, and an increased amount of carbonic acid gas and water exhaled by the breath. The oxygen which is absorbed from the air into the blood is stored in the red corpuscles of the latter, by which it is carried to every part of the body. The venous blood which returns from every portion of the system comes back as a dark crimson, instead of being bright scarlet like the arterial blood. It contains carbonic acid, and returns it to the lungs, where it is exhaled by the breath. The oxygen is necessary to existence, while the carbonic acid is injurious. The effect of exercise, then, in any form, is thus to distribute healthy blood more rapidly through the system, while it removes the injurious matters quite as speedily. The effect of active exercise on the heart, as it is well known, is to make it beat faster; by this the blood is driven through the body at a quicker rate than usual. Sometimes, when the effort is unusually severe, there is a disturbance of the regular balance between the heart and the lungs. There is thus produced an irregular or unequal action of the former, causing what is known as "loss of wind," which is, however, soon restored by resting. There is an excessive flow of blood to the surface of the body, causing it to redden, and at the same time the perspiration is greatly increased. It is on account of this latter moisture opening up the pores of the skin that the good effects of exercise are chiefly due. The perspiration consists mainly of water containing different salts and organic matters. It is found by experiment that the amount of water passing through the lungs and skin is usually doubled even with moderate exertion. The result of moderate exercise in benefiting the nervous system is well known, and the effect of a gentle walk in making the ideas flow through the brain is a matter of common observation. At the same time, it must be borne in mind that exercise, when carried to the verge of fatigue, compels inactivity of the brain for a time, since Nature must have repose. But when carried out in moderation with a view of improving the condition of the body, it conduces to the salubrity of the brain as well, for the latter organ shares in the health of the former. The only thing to guard against is irregular and fitful doses; thus it is far better to take a little in moderation daily, than to attempt to make one day's exercise suffice for the rest of the week.