so small
that the total sum of its nervous energy must be very slight. The total
energy of this minute machine is so slight that it can not be detected
by our comparatively rough instruments of measurement.

In short, all evidence goes to show that the nerve impulse is a form of
motion, and hence of energy, correlated with other forms of physical
energy. The nerve is, however, a very delicate machine, and its total
amount of energy is very small. A tiny watch is a more delicate machine
than a water-wheel, and its actions are more dependent upon the accuracy
of its adjustment. The water-wheel may be made very coarse and yet be
perfectly efficacious, while the watch must be fashioned with extreme
delicacy. Yet the water-wheel transforms vastly more energy than the
watch. It may drive the many machines in a factory, while the watch can
do no more than move itself. But who can doubt that the watch, as well
as the water-wheel, is governed by the law of the correlation of forces?
So the nervous system of the living machine is delicately adjusted and
easily put out of order, and its action involves only a small amount of
energy; but it is just as truly subject to the law of the conservation
of energy as is the more massive muscle.

_Sensations_.--Pursuing this subject further, we next notice that it is
possible to trace a connection between physical energy and _sensations_.
Sensations are excited by certain external forms of motion. The living
machine has, for example, one piece of apparatus capable of being
affected by rapidly vibrating waves of air. This bit of the machine we
call the ear. It is made of parts delicately adjusted, so that vibrating
waves of air set them in motion, and their motion starts a nervous
stimulus travelling along the auditory nerve. As a result this apparatus
will be set in motion, and an impulse sent along the auditory nerve
whenever that external type of motion which we call sound strikes the
ear. In other words, the ear is a piece of apparatus for changing air
vibrations into nervous stimulation, and is therefore a machine.
Apparently the material in the ear is like a bit of gunpowder, capable
of being exploded by certain kinds of external excitation; but neither
the gunpowder nor the material in the ear develops any energy other than
that in it at the outset. In the same way the optic nerve has, at its
end, a bit of mechanism readily excited by light vibrations of the
ether, and hence the optic