First: Surplus nitrogen is usually cast from the body as waste matter when it is not deposited as muscle tissue by proper exercise. If the diet is balanced, regular exercise will add this nitrogenous substance to the muscle-cells far beyond normal growth, thus causing an actual increase in the size and the number of fibres. [Sidenote: Brain and nerve force increased] Second: A second benefit derived from muscle activity is the consequent change that occurs in brain and in nerve activity. There are certain cells in the brain and in the nervous system which control the movements of the muscles. When these cells are not used, they degenerate, but their use in exercise is not only beneficial in developing a well-rounded nervous mechanism, but also in strengthening the brain-cells that are used in intellectual work. [Sidenote: Blood circulation increased] Third: A third and perhaps most important of all the benefits to be derived from exercise is the general increase in the circulation of the blood. The muscles form a larger proportion of the body-weight than any other group of organs. When general exercise involving the larger muscles is participated in, the demand for food material in this particular muscular tissue is so great as to cause a notable increase in the strength and in the rapidity of the heart beat, and consequent deep breathing. This acceleration of the circulation continues long after the exercise has ceased, thus replenishing and building up the muscles. As a result of the better circulation of the blood, all organs receive an increased blood-supply, and every part of the body shares in the general improvement. This explains why one can do better brain work, or digest food with greater ease after taking moderate exercise. [Sidenote: Evil effect of long-continued exercise] Exercise is constructive up to the point of fatigue, but beyond that point it is destructive. The waste products of all cell-metabolism are harmful and poisonous. When exercise is long continued, the waste matter accumulating therefrom weakens or poisons the cells that secrete them. [Sidenote: Different forms of exhaustion] The products of cell-metabolism are of two classes, and each class has different effects. The first is due to oxidation. A runner, who falls exhausted from shortness of breath, has simply been suffocated by the excess of carbon dioxid in his muscles. After the breath is regained, or, in other words, after the body has had time to throw off the carbon dioxid, the runner is in nearly as good condition as before. A more lasting and serious form of exhaustion is due to the accumulation of nitrogenous decomposition products, which, not being in a gaseous form, cannot be thrown off from the lungs, and hence are not as rapidly or as easily removed from the tissues. The presence in the tissue of these waste-products is the cause of extreme weakness and fatigue. [Sidenote: The causes of soreness or stiffness of the muscles]