ed 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
transmiss