igue is a chemical affair. It is the
result of recent effort,--physical, mental, or emotional,--and is the
sum of sensations arising from the presence of waste material in the
muscles and the blood. The whole picture becomes clear if we think of
the body as a factory whose fires continuously burn, yielding heat and
energy, together with certain waste material,--carbon dioxide and ash.
Within man's body the fuel, instead of being the carbon of coal is the
carbon of glycogen or animal starch, taken in as food and stored away
within the cells of the muscles and the liver. The oxygen for
combustion is continuously supplied by the lungs. So far the factory
is well equipped to maintain its fires. Nor does it fail when it comes
to carrying away waste products. Like all factories, the body has its
endless chain arrangement, the blood stream, which automatically picks
up the debris in its tiny buckets--the blood-cells and serum--and
carries it away to the several dumping-grounds in lungs, kidneys,
intestines, and skin.

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 thou