as fain, for the most
part, as were his predecessors in the century before, to have recourse to
such vague terms as "fermentation" and the like; to-day our treatises on
physiology are largely made up of precise and exact expositions of the
play of physical agencies and chemical bodies in the living organisms. He
made use of the words "vital force" or "vital principle," not as an
occasional, but as a common, explanation of the phenomena of the living
body. During the present century, especially during its latter half, the
idea embodied in those words has been driven away from one seat after
another; if we use it now when we are dealing with the chemical and
physical events of life, we use it with reluctance, as a _deus ex machina_
to be appealed to only when everything else has failed.

Some of the problems--and those, perhaps, the chief problems--of the
living body have to be solved, neither by physical nor by chemical
methods, but by methods of their own. Such are the problems of the nervous
system. In respect to these the men of 1799 were on the threshold of a
pregnant discovery. During the latter part of this nineteenth century,
especially during its last quarter, the analysis of the mysterious
processes in the nervous system, and especially in the brain, which issue
as feeling, thought, and the power to move, has been pushed forward with a
success conspicuous in its practical, and full of promise in its
theoretical, gains. That analysis may be briefly described as a following
up of threads. We now know that what takes place along a tiny thread which
we call a nerve fibre differs from that which takes place along its fellow
threads, that differing nervous impulses travel along different nervous
fibres, and that nervous and physical events are the outcome of the
clashing of nervous impulses as they sweep along the closely woven web of
living threads of which the brain is made. We have learned by experiment
and by observation that the pattern of the web determines the play of the
impulses, and we can already explain many of the obscure problems, not
only of nervous disease, but of nervous life, by an analysis which is a
tracking out of the devious and linked path of nervous threads. The very
beginning of this analysis was unknown in 1799. Men knew that nerves were
the agents of feeling and of the movements of muscles; they had learned
much about what this part or that part of the brain could do; but they did
not know th