must do so by finding some way of measuring nervous energy and comparing it with the latter.  This has been very difficult, for we have no way of measuring a nervous impulse directly.  In the larger experiments upon the income and outgo of the body, in the respiration apparatus mentioned above, nervous phenomena apparently leave no trace.  So far as experiments have gone as yet, there is no evidence of an expenditure of extra physical energy when the nervous system is in action.  This is not surprising, however, for this apparatus is entirely too coarse to measure such delicate factors.

That there is a correlation between nervous energy and physical energy is, however, pretty definitely proved by experiments along different lines.  The first step in this direction was to find that a nervous stimulus can be measured at least indirectly.  When the nerve is stimulated there passes from one end to the other an impulse, and the rapidity with which it travels can be accurately measured.  When such an impulse reaches the brain it may give rise to a conscious sensation, and a somewhat definite estimation can be made of the amount of time required for this.  The periods are very short, of course, but they are not instantaneous.  The nervous impulse, can be studied in still other ways.  We find that the impulse can be started by ordinary forms of energy.  A mechanical shock, a chemical or an electrical shock will develop nervous energy.  Now these are ordinary forms of physical energy, and if, when they are applied to a nerve, they give rise to a nervous stimulus, the inference is certainly a legitimate one that the nerve is simply a bit of machinery adapted to the conversion of certain kinds of physical energy into nervous energy.  If this is the case, then it is necessary to regard nervous energy as correlated with other forms of energy.

Other facts point in the same direction.  Not only can the nervous stimulus be developed by an electric shock, but the strength of the stimulus is within certain limits proportional to the strength of the shock which produces it.  Again, not only is it found that an electrical shock can develop a nervous stimulus, but conversely a nervous stimulus develops electrical energy.  In ordinary nerves, even when not active, 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.