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mechanical power. The mechanical power is employed for muscular
work. The heat is used to keep the body warm, and when more is
generated than is needed for that purpose it is wasted, as in the case of
the engine.
One important difference between the human machine and the steam
engine is that the former is self-building, self-repairing, and self-regulating.
Another is that the material of which the engine is built is '
very different from that which it uses for fuel, but part of the material
which serves the body for fuel also builds it up and keeps it in
repair. Furthermore, the body can use its own substance for fuel.
This the steam engine gan not do. The steam engine and the body are
alike in that both convert the fuel into heat and mechanical power.
They differ in that the body uses the same material for fuel as for
building and also consumes its own material for fuel. In the use of
fuel the body is much more economical than any engine.
But the body is more than a machine. It has not simply organs to
build and keep in repair and supply with energy; it has a nervous
organization; it has sensibilities; and there are the higher intellectual
and spiritual faculties. The right exercise of these depends upon the
right nutrition of the body.
The chief uses of food, then, are two: (1) To form the material of
the body and repair its wastes, and (2) to yield heat to keep the body
warm and furnish muscular and other power for the work it has to
do. In forming the tissues and the fluids of the body the food serves
for building and repair. In yielding heat and power it serves as fuel.
If more food is eaten than is needed, more or less of the surplus
may be and sometimes is stored in the body, chiefly in the form of
fat. The fat in the body forms a sort of reserve supply of fuel and
is burned in the place of food. When the work is hard or the food
supply is low the body draws upon this store of fat and grows lean.
PROTEIN AS BUILDING MATERIAL.
The principal tissue formers are the protein compounds, especially
the albuminoids. These make the framework of the body. They
build up and repair the nitrogenous materials, as the muscles and tendons,
and supply the albuminoids of the blood, milk, and other fluids.
The albuminoids of food are transformed into the albuminoids and
gelatinoids of the body. Muscle, tendon and cartilage, bone and skin,
the corpuscles of the blood, and the casein of milk are made of the
albuminoids of food. The albuminoids are sometimes called "flesh
formers" or "muscle formers," because the lean flesh, the muscle, is
made from them, though the term is inadequate, as it leaves out of
account the energy-furnishing function of protein. The gelatinoids of
food, such as the finer particles of tendon and the gelatin, which are
