the physiology of man is tolerably well known, that
of other animals is less understood the farther we get away from man and
his allies. For this reason most of our knowledge of the living body as
a machine must be derived from the study of man. This is, however,
fortunate rather than otherwise. In the first place, it enables us to
proceed from the known to the unknown; and in the second place, more
interest attaches to the problem as connected with human physiology than
along any other line. In our discussion, therefore, we shall refer
chiefly to the physiology of man. If we find that the functions of human
life are amenable to a mechanical explanation we cannot hesitate to
believe that this will be equally true of the lower orders of nature.
For similar reasons little reference will be made to the mechanism of
plant life. The structure of the plant is simpler and its activities are
much more easily referable to mechanical principles than are those of
animals. For these reasons it will only be necessary for us to turn our
attention to the life activities of the higher animals.

==What is a Machine?==--Turning now to our more immediate subject of the
accuracy of the statement that the body is a machine, we must first ask
what is meant by a machine? A brief definition of a machine might be as
follows: _A machine is a piece of apparatus so designed that it can
change one kind of energy into another for a definite purpose_. Energy,
as already noticed, is the power of doing work, and its ordinary active
forms are heat, motion, electricity, light, etc.; but it may be in a
passive or potential form, and in this form stored within a chemical
molecule. These various forms of energy are readily convertible into
each other; and any form of apparatus designed for the purpose of
producing such a conversion is called a machine. A dynamo is thus a
machine so adjusted that when mechanical motion is supplied to it the
energy of motion is converted into electricity; while an electromotor,
on the other hand, is a piece of apparatus so designed that when
electricity is applied to it, it is converted into motion. A steam
engine, again, is designed to convert potential or passive energy into
active energy. Potential energy in the form of chemical composition
(coal) is supplied to the engine, and this energy is first liberated in
the active form of heat and then is converted into the motion of the
great fly-wheel. In all these cases there is