all the oxygen within their reach.  The result is that in these tissues the oxygen pressure is very low, and when the oxygen-laden haemoglobin reaches them the association of the haemoglobin with oxygen is at once broken up and the oxygen set free in the tissue.  It passes at once to the lymph, from which the active tissues seize it for the purpose of carrying on the oxidizing processes of the body.  This whole matter of supplying the body with oxygen is thus fundamentally a chemical one, controlled by chemical laws.

Removal of Waste.—­The next step in this life process is one of difficulty.  After the food and oxygen have reached the tissues it is seized by the living cell.  The food material is now oxidized by the oxygen and its latent energy is liberated, and appears in the form of motion or heat or some other vital function.  Herein is the really mysterious part of the life process; but for the present we will overlook the mystery of this action, and consider the results from a purely material standpoint.

In a steam engine the fundamental process by which the latent energy of the fuel is liberated is that of oxidation.  The oxygen of the air unites with the chemical elements of the fuel, and breaks up that fuel into simple compounds—­which may be chiefly considered as three—­carbonic dioxide (CO_{2}), water (H_{2}O), and ash.  The energy contained in the original compound can not be held by these simpler bodies, and it therefore escapes as heat.  Just the same process, with of course difference in details, is found in the living machine.  The food, after reaching the living cell, is united with the oxygen, and, so far as chemical results are concerned, the process is much the same as if it occurred outside the body.  The food is broken into simpler compounds and the contained energy is liberated.  The energy is, by the mechanism of the machine, changed into motion or nervous impulse, etc.  The food is broken into simple compounds, which are chiefly carbonic dioxide, water, and ash; the ash being, however, quite different from the ash obtained from burning coal.  Now the engine must have its chimney to remove the gases and vapours (the CO_{2} and H_{2}O) and its ashpit for the ashes.  In the same way the living machine has its excretory system for removing wastes.  In the removal of the carbonic acid and water we have to do once more with the respiratory system, and the process is simply a repetition of the story of gas diffusion, chemical union, and osmosis.  It is sufficient here to say that the process is just as simple and as easily explained as those already described.  The elimination of these wastes is simply a problem of chemistry and mechanics.