ines to-day utilize a somewhat larger part, but
most of them utilize less than one-tenth. The experiments with the
living body in the respiration apparatus above described, give a means
of determining the proportion of the energy furnished in the form of
food which can be utilized in the form of motive force. This figure
appears to be decidedly larger than that obtained by any machine yet
devised by man.

The conclusion of the matter up to this point is then clear. If we leave
out of account the phenomena of the nervous system, which we shall
consider presently, _the general income and outgo of the body as
concerns matter and energy is such that the body must be regarded as a
machine, which, like other machines, simply transforms energy without
creating or destroying it. To this extent, at least, animals conform to
the law of the conservation of energy and are veritable machines_.

==Details of the Action of the Machine.==--We turn next to some of the
subordinate problems concerning the details of the action of the living
machine. We have a clear understanding of the method of action of a
steam engine. Its mechanism is simple, and, moreover, it was designed by
human intelligence. We can understand how the force of chemical affinity
breaks up the chemical composition of the coal, how the heat thus
liberated is applied to the water to vapourize it; how the vapour is
collected in the boiler under pressure; how this pressure is applied to
the piston in the cylinder, and how this finally results in the
revolution of the fly-wheel. It is true that we do not understand the
underlying forces of chemism, etc., but these forces certainly exist and
are the foundation of science. But the mechanism of the engine is
intelligible. Our understanding of it is such that, with the forces of
chemistry and physics as a foundation, we can readily explain the
running of the machine. Our next problem, therefore, is to see if we
can in the same way reach an understanding of the phenomena of the
living machine. Can we, by the use of these same chemical and physical
forces, explain the activities taking place in the living organism? Can
the motion of the body, for example, be made as intelligible as the
motion of the steam engine?

==Physical Explanation of the Chief Vital Functions.==--The living machine
is, of course, vastly more complicated than the steam engine, and there
are many different processes which must be considered separately. The