An adult cannot race and scamper about like a child who plays for hours together, and a disturbance of the heart’s action brought about by strenuous exercise to the point of fatigue of the heart is not so quickly compensated. On the other hand, severe exercises of strength and endurance are not so apt to prove injurious after the completion of growth as they are in the growing youth. Up to the age of eighteen years no feats of strength or of endurance should be attempted. From eighteen to thirty years is the period of life when any kind of athletic exercise can be taken, not only without any injurious, but with beneficial, results. After the period of youth new conditions begin to make themselves felt, and more care must be exercised in the demands made upon the heart. In some persons obesity sets in before they have reached thirty and impedes the action of the heart. About the fortieth year the walls of the arteries begin to lose their elasticity, they become more rigid, and chalk salts are deposited in them. Golf and lawn-tennis are now excellent. And now it is most essential that exercise be kept up; the heart must still be trained and practised. The fibers of every muscle degenerates when their work is reduced to a minimum. Proper health without proper breathing is a physical impossibility. It is necessary that those portions of the lungs which do take part in ordinary breathing, and which would atrophy from lack of use, should be fully developed and kept ready for suitable exercise. As soon as the lungs grow weary and the power of breathing is exhausted, the most powerful muscles of the body give way. The pleasure of vigorous walking, especially in mountainous places, is alone for her who can respond easily and readily to the enormously increased demands on the power of the respiration. The direct result of exercise is an increased demand for oxygen by the tissues, and, to meet this demand, respiration is deepened and quickened, and the beat of the heart is more rapid and more forcible. But the phenomena of increased breathing power and increased heart action benefit other parts of the body. At the commencement of an exercise the contraction of the voluntary muscles called into action compresses the blood-vessels, and impels the venous blood actively toward the heart, which, thus stimulated, contracts vigorously, and propels the blood in increased quantity toward the lungs. Stimulated by the pressure of a large amount of venous blood, the inspiratory muscles contract and elevate the bony structure of the chest, the diaphragm pushes down the abdominal contents, and the air rushes in to fill the vacuum thus produced and to supply the oxygen necessary for the purification of the blood. Supplied with this life-giving element, the blood is returned to the heart, to be distributed again throughout the system, and to restore the loss incurred in the original muscular movements.