==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.  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