56 THE STORY OF LIFE'S MECHANISM. 



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 stimu- 

 lus develops electrical energy. In ordinary nerves, 

 even when not active, slight electric currents can 

 be detected. They are extremely slight, and re- 

 quire 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. 



