IS THE BODY A MACHINE ? 43 



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 im- 

 pulse 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 le- 

 gitimate one that the nerve is simply a bit of ma- 

 chinery 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 cur- 

 rents can be detected. They are extremely slight, 

 and require the most delicate instruments for 

 their detection. Now when a nerve is stimulated 



