THE MECHANICS OF THE LIVING MACHINE 315 



nervous impulse. Now, if forms of physical energy applied to 

 a nerve are capable of giving rise to a nerve stimulus, the 

 inference is certainly a legitimate one that the nerve is simply 

 a bit of machinery which converts one kind of energy into 

 another, i. e., converts physical energy into nervous energy. 

 If this be the case, of course it is necessary for us to regard 

 nervous force as one of the correlated forms of energy. 



Other facts point in the same direction. Not only can the 

 nerve stimulus be developed by an electric shock, but the 

 strength of the stimulus is, within certain limits, proportional 

 to the strength of the shock producing it. Conversely, we also 

 find that a nerve stimulus produces electrical energy. In an 

 ordinary nerve, even when it is not active, there are slight 

 electric currents that can be detected by very delicate appa- 

 ratus. If the nerve is stimulated, these electric currents are 

 immediately affected in such a way that they may be increased 

 or decreased in intensity. These variations in intensity are 

 sufficient to be visible by delicate apparatus, and by using a 

 galvanometer we can actually measure the passage of an im- 

 pulse passing along a nerve like a wave, and can approximately 

 determine the shape of the wave. 



Since the nervous impulse can be started by some other 

 form of energy, and since in turn it can modify ordinary forms 

 of energy, we cannot avoid the conclusion that the nervous 

 impulse is a special form of energy developed within the nerves. 

 It is possibly a form of wave motion, peculiar to the nerve 

 substance, but correlated with and developed by other types 

 of energy. This of course would make the nerve fiber a simple 

 bit of machinery. 



If this conclusion is correct, it will follow that whenever a 

 nerve impulse passes over a nerve a certain portion of the food 

 supply in the nerve must be broken to pieces to liberate energy, 

 and this would also be accompanied by the elimination of 

 carbon dioxid and heat. But although careful experiments 

 have been made, it is as yet impossible to detect any rise in 



