In the removal of the ash, however, we have something more, for here again we are brought up against the vital action of the cell.  This ash takes chiefly the form of a compound known as urea, which finds its way into the general circulatory system.  From the blood it is finally removed by the kidneys.  In the kidneys are a large number of bits of living matter (kidney cells), which have the power of seizing hold of the urea as the blood is flowing over them, and after thus taking it out of the blood they deposit it in a series of tubes which lead to the bladder and hence to the exterior.  The bringing of this ash to the kidney cell is a mechanical matter, based simply upon the flow of the blood.  The seizing of the urea by the kidney cell is a vital phenomenon which we must waive for the moment.

Up to this point in the analysis there has been no difficulty, and no one can fail to agree with the conclusions.  The position we reach is as follows:  So far as relates to the general problems of energy in the universe the body is a machine.  It neither creates nor destroys energy, but simply transforms one form into another.  In attempting to explain the action of the machine, we find that for the functions thus far considered (sometimes called the vegetative functions) the laws of chemistry and physics furnish adequate explanation.

We must now look a little further, and question some of the functions the mechanical nature of which is less obvious.  The whole operation thus far described is under the control of the nervous system, which acts somewhat like the engineer of an engine.  Can this phase of living activity be included within the conception of the body as a machine?

Nervous System.—­When we come to try to apply mechanical principles to the nervous system, we meet with what seems at first to be no thoroughfare.  While dealing with the grosser questions of chemical compounds, heat, and motion, there is little difficulty in applying natural laws to the explanation of living phenomena.  But the problem with the nervous system is very different.  It is only to-day that we are finding that the problem is open to study, to say nothing of solution.  It is true that mental and other nervous phenomena have been studied for a long time, but this study has been simply the study of these phenomena by themselves without a thought of their correlation with other phenomena of nature.  It is a matter of quite recent conception that nervous phenomena have any direct relation to the other realms of nature.

Our first question must be whether we can find any correlation between nervous energy and other types of energy.  For our purpose it will be convenient to distinguish between the phenomena of simple nervous transmission and the phenomena of mental activity.  The former are the simpler, and offer the greatest hope of solution.  If we are to find any correlation between nervous energy and other physical energy, we