Besides the products of combustion, there are always to be washed away some broken-down particles from the tissues themselves, which, like all machinery, are being continuously worn out and repaired. By chemical tests in the laboratory, the physiologist finds that a muscle which has recently been in violent exercise contains among other things carbon dioxid, urea, creatin, and sarco-lactic acid, none of which are found in a rested muscle. Since all this debris is acid in reaction and since we are "marine animals," at home only in salt water or alkaline solution, the cells must be quickly washed of the fatigue products, which, if allowed to accumulate, would very soon poison the body and put out the fires. =No Back Debts.= The human machine is regulated to carry away its fatigue products as fast as they are made, with but slight lagging behind that is made good in the hours of sleep, when bodily activities are lessened and time is allowed for repair. Unless the body is definitely diseased, it virtually never carries over its fatigue from one day to another. In the matter of fatigue, there are no old debts to pay. Nature renews herself in cycles, and her cycle is twenty-four hours,--not nine or ten months as many school-teachers seem to imagine, or eleven months as some business men suppose. In order to make assurance doubly sure, many set apart every seventh day for a rest day, for change of occupation and thought, and for catching up any slight arrears which might exist. But the point is that a healthy body never gets far behind. If through some flaw in the machine, waste products do pile up, they destroy the machine. If the heart leaks or the blood-cells fail in their carrying-power, or if lungs, kidneys or skin are out of repair, there is sometimes an accumulation of fatigue products which poisons the whole system and ends in death. But the person with tuberculosis or heart trouble does not usually allow this to happen. The body incapacitated by disease limits its activities as closely as possible within the range of its power to take care of waste matter. Even the sick body does not carry about its old toxins. The man who had not eliminated the poisons of a month-old effort would not be a tired man. He would be a dead man.