active vital property of the
gland-cells in secreting. In studying absorption we understand the
process until we come to what we have called the vital powers of the
absorptive cells of the alimentary canal. The circulation is
intelligible until we come to the beating of the heart and the
contraction of the muscles of the blood-vessels. Excretion is also
partly explained, but here again we finally must refer certain processes
to the vital powers of active cells. And thus wherever we probe the
problem we find ourselves able to explain many secondary problems, while
the fundamental ones we still attribute to the vital properties of the
active tissues. Why a muscle contracts or a gland secretes we have
certainly not yet answered. The relation of the actions to the general
problems of correlation of force is simple enough. That a muscle is a
machine in the sense of our definition is beyond question. But the
problem of _why_ a muscle acts is not answered by showing that it
derives its energy from broken food material. There are plainly still
left for us a number of fundamental problems, although the secondary
ones are soluble.

What can we say in regard to these fundamental vital powers of the
active tissues? Firstly, we must notice that many of the processes which
we now understand were formerly classed as vital, and we only retain
under this term those which are not yet explained. This, of course,
suggests to us that perhaps we may some day find an explanation for all
the so-called vital powers by the application of simple physical forces.
Is it a fact that the only significance to the term vital is that we
have not yet been able to explain these processes to our entire
satisfaction? Is the difference between what we have called the
secondary processes and the primary ones only one of degree? Is there a
probability that the actions which we now call vital will some day be as
readily understood as those which have already been explained?

Is there any method by which we can approach these fundamental problems
of muscle action, heart beat, gland secretion, etc.? Evidently, if this
is to be done, it must be by resolving the body into its simple units
and studying these units. Our study thus far has been a study of the
machinery of the body as a whole; but we have found that the various
parts of the machine are themselves active, that apart from the action
of the general machine as a whole, the separate parts have vital power