ive,
slight electric currents can be detected. They are extremely slight, and
require the most delicate instruments for their detection. Now when a
nerve is stimulated these currents are immediately affected in such a
way that under proper conditions they are increased in intensity. The
increase is sufficient to make itself easily seen by the motion of a
galvanometer. The motion of the galvanometer under these conditions
gives a ready means of studying the character of the nervous impulse. By
its use it can be determined that the nerve impulse travels along the
nerve like a wave, and we can approximately determine the length and
shape of the wave and its relative height at various points.

Now what is the significance of all these facts for our discussion?
Together they point clearly to the conclusion that nervous energy is
correlated with other forms of physical energy. Since the nervous
stimulus is started by other forms of energy, and since it can, in turn,
modify ordinary forms of energy, we can not avoid the conclusion that
the nervous impulse is only a special form of energy developed within
the nerve. It is a form of wave motion peculiar to the nerve substance,
but correlated with and developed from other types of energy. This, of
course, makes the nerve simply a bit of machinery.

If this conclusion is true, the development of a nerve impulse would
mean that a certain portion of food is broken to pieces in the body to
liberate energy, and this should be accompanied by an elimination of
carbonic dioxide and heat. This is easily shown to be true of muscle
action. When we remove a muscle from the body it may remain capable of
contracting for some time. By studying it under these conditions we find
that it gives rise to carbonic dioxide and other substances, and
liberates heat whenever it contracts. As already noticed, in the
respiration experiments, whenever the individual experimented upon
makes any motions, there is an accompanying elimination of waste
products and a development of heat. But this does not appear to be
demonstrable for the actions of the nervous system. Although very
careful experiments have been made, it has as yet been found impossible
to detect any rise in temperature when a nerve impulse is passing
through a nerve, nor is there any demonstrable excretion of waste
products. This would be a serious objection to the conception of the
nerve as a machine were it not for the fact that the nerve is