"Handbook of School Gymnastics of the Swedish Systems,"
by the same author.

Chapter V.

Food and Drink.

98. Why we need Food. The body is often compared to a steam-engine in
good working order. An engine uses up fuel and water to obtain from them
the energy necessary to do its work. So, we consume within our bodies
certain nutritious substances to obtain from them the energy necessary for
our activities. Just as the energy for the working of the engine is
obtained from steam by the combustion of fuel, so the energy possessed by
our bodies results from the combustion or oxidation within us of the food
we eat. Unless this energy is provided for the body it will have but
little power of doing work, and like an engine without steam, must soon
become motionless.

99. Waste and Repair. A steam-engine from the first stroke of its
piston-rod begins to wear out, and before long needs repair. All work
involves waste. The engine, unless kept in thorough repair, would soon
stop. So with our bodies. In their living cells chemical changes are
constantly going on; energy, on the whole, is running down; complex
substances are being broken up into simpler combinations. So long as life
lasts, food must be brought to the tissues, and waste products carried
away from them. It is impossible to move a single muscle, or even to think
for one moment, without some minute part of the muscular or brain tissue
becoming of no further use in the body. The transformation of dead matter
into living tissue is the ever-present miracle which life presents even in
its lowest forms.

In childhood the waste is small, and the amount of food taken is more
than sufficient to repair the loss. Some of the extra food is used in
building up the body, especially the muscles. As we shall learn in Chapter
VIII., food is also required to maintain the bodily heat. Food, then,
is necessary for the production of energy, for the repair of the body, for
the building up of the tissues, and for the maintenance of bodily heat.

100. Nature of the Waste Material. An ordinarily healthy person
passes daily, on an average, by the kidneys about 50 ounces of waste
material, of which 96 per cent is water, and from the intestines, on an
average, 5-1/2 ounces, a large proportion of which is water. By the skin,
in the shape of sweat and insensible perspiration, there is cast out about
23 ounces, of which 99 per cent is water; and by the lungs about 34
ounces, 10