The important thing to bear in mind in both work and play, in athletic training, and in life, is that this work must be kept easily within the powers of the heart and of the other muscles, and must be increased gradually, and never allowed to go beyond a certain point, or it becomes injurious, instead of beneficial; hurtful, instead of helpful. Over-work in the shop or factory, overtraining in the gymnasium or on the athletic field, both fall first and heaviest upon the heart. Importance of Food, Air, and Exercise. At the same time, the system must be kept well supplied through the stomach with the raw material both for doing this work and for building up this new muscle. When anyone, in training for an event, gets "stale," or overtrained, and loses his appetite and his sleep, he had better stop at once, for that is a sign that he is using more energy than his food is able to give him through his stomach; and the stomach has consequently "gone on a strike." How to Avoid Heart Overstrain and Heart Disease. The way, then, to avoid overstrain and diseases of the heart and blood vessels is:-- First, to take plenty of exercise, but to keep that exercise within reasonable limits, which, in childhood, ought to be determined by a school physician, and in workshops and factories by a state factory physician. Second, to take that exercise chiefly in the open air, and as much of it as possible in the form of play, so that you can stop whenever you begin to feel tired or your heart throbs too hard--in other words, whenever nature warns you that you are approaching the danger line. Third, to keep yourself well supplied with plenty of nutritious, wholesome, digestible food, so as to give yourself, not merely power to do the work, but something besides to grow on. Fourth, to avoid poisonous and hurtful things like the toxins of infectious diseases; and alcohol, tobacco, and other narcotics, which have a harmful effect upon the muscles, valves, or nerves of your heart, or the walls of your blood vessels. Fortunately, the heart is so wonderfully tough and elastic, and can repair itself so rapidly, that it usually takes at least two, and sometimes three, causes acting together, to produce serious disease or damage. For instance, while muscular overwork and overstrain alone may cause serious and even permanent damage to the heart, they most frequently do so in those who are underfed, or badly housed, or recovering from the attack of some infectious disease. While the poisons of rheumatism and alcohol will alone cause serious damage to the valves of the heart and walls of the blood vessels, yet they again are much more liable to do so in those who are overworked, or underfed, or overcrowded.