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 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.