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. There
is not space in a work of this size to consider them all carefully, but
we may select a few of the vital functions as illustrations of the
method which is pursued. It will be assumed that the fundamental
processes of human physiology are understood by the reader, and we shall
try to interpret some of them in terms of chemical and physical force.

_Digestion._--The first step in this transformation of fuel is the
process of digestion. Now this process of digestion is nothing
mysterious, nor does it involve any peculiar or special forces.
Digestion of food is simply a chemical change therein. The food which is
taken into the body in the form of sugar, starch, fat or protein, is
acted upon by the digestive juices in such a way that its chemical
nature is slightly changed. But the changes that thus occur are not
peculiar to the living body, since they will take place equally well in
the chemist's laboratory. They are simply changes in the molecular
structure of the food material, and only such changes as are simple and
familiar to the chemist. The forces which effect the change are
undoubtedly those of chemical affinity. The only feature of the process
which is not perfectly intelligible in terms of chemical law is the
nature of the digestive juices. The digestive fluids of the mouth and
stomach contain certain substances which possess a somewhat remarkable
power, inasmuch as they are able to bring about the chemical changes
which occur in the digestion of food. An example will make this clearer.