44 THE STORY OF THE LIVING MACHINE. 



these currents are immediately affected in such a 

 way that under proper conditions they are in- 

 creased in intensity. The increase is sufficient to 

 make itself easily seen by the motion of a galva- 

 nometer. 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 approxi- 

 mately determine the length and shape of the 

 wave and its relative height at various points. 



Now what is the significance of all these facts 

 for our discussion ? Together they point clearly 

 to the conclusion that nervous energy is corre- 

 lated 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 study- 

 ing 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 



