However, there are certain conditions having their origin in the will and the excitement of mental forces, where the heart is pushed beyond its self-control. Such cases sometimes occur in college boat races――the four-mile races. It is not so much the mere muscular efforts of the contestants as it is the intense mental excitement accompanying the effort. No youth of a highly-strung, nervous temperament should row in these races――no matter what his muscular strength is. The heart, though making contractions at the rate of seventy-two times a minute, is able to continue its work without fatigue throughout the life of the individual. Each contraction of this wonderful muscle is followed by an interval of rest, during which the cells recuperate. Push the heart-beats to a very rapid rate and we approach the danger-line at which fatigue products cannot be replaced by fresh cells; the intervals of rest are not sufficient. The same conditions exist in every muscle. This is the reason why we often see immediate or ultimate collapse at the finish of a four-mile boat race or a quarter-mile run. Such a collapse may be followed by irremediable loss of health. You should remember all through your life that each member of the body, in the very act of living, produces poison to itself. When this poison accumulates faster than it can be thrown off, which always occurs unless the muscle can have an interval of rest, then will come fatigue, which is only another expression for poisonous infection――real, genuine poison. If the muscle is given an interval of rest, so that the cell can give off its waste product to keep pace with the new productions, the muscle will then liberate energy for a long time. This latter condition is what we call endurance. When you say a boy has long endurance it simply means that all his organs are in perfect working order; that they repair as soon as they waste. Such a condition kept up means a long life and a healthy one. And you can all obtain this state, as I shall later on show you. The power and endurance of the human machine are limited according to our understanding of the above facts. But another important bit of knowledge is necessary to have if we wish to avoid ruining our physical energy: that is, to recognize the necessity of starting the human machine slowly. Like any other ponderous and intricate machine, the body requires time to get in harmonious working order. What would you think of an engineer who started his engine off with a jump at high speed? What would happen to a big engine if the throttle were pulled wide open at the instant of starting?