i88 POPULAR SCIENCE MONTHLY.
capital must not be expended at a greater rate than it can be re- placed; if it is expended at a greater rate, fatigue commences, and a continuance of this expenditure results in physical bankruptcy. The muscle is continually undergoing change of material. The minute substances which make up the muscle, and whose very ac- tions keep it alive, are being continually cast off, fresh substances taking their place. The cast-off material is the fatigue poison. Without muscle rest this dead, poisonous detritus can not be re- placed fast enough by the new products, and the result is an im- poverished capital of potential elements. This does not apply only to the muscle in active use up to this point, but to all muscles of the body.
The energy products of food are delivered up to the muscle by the blood, and this fluid picks up and carries away the cast-off dead substances of the muscle. " If the working muscle has taken ma- terial from the blood, this material is lost to all the rest; and if the working muscle has given off to the blood poisonous material, this is added to the other parts " (Lombard; Howell's Physiology). These latter, the fatigue products, are only gradually eliminated from the blood. It will now be recognized that to keep on the right side of the danger line in exercise the muscle must have short intervals of rest. Nature so well understood the proneness of man not to heed advice that she placed the action of one muscle beyond his control. This muscle is so internally constructed and adjusted that it has its regular jDcriods of rest, and only in disorder of the body can its expenditure be raised beyond its means. This muscle, 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 muscle is followed by an interval of rest, during which the cells recuperate. Push con- tinually the heart beats to a very rapid rate and we approach the danger point at which the fatigue products can not be replaced by fresh cells; the intervals of rest are not sufficient. The same con- dition exists in every muscle. It is in the extreme rapid exercise, such as sprinting and certain phases of bicycle racing, that we often see either immediate or ultimate collapse followed by irremediable loss of health.
It should be impressed upon all young persons that during life each member of the body, in the very act of living, produces poison to itself. When this poison accumulates faster than it can be elimi- nated, which always occurs unless the muscle has an interval of rest, then will come fatigue, which is only another expression for toxic infection. 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 produc-