58 THE STORY OF LIFE'S MECHANISM. 



there any demonstrable excretion of waste pro- 

 ducts. This would be a serious objection to the 

 conception of the nerve as a machine were it not 

 for the fact that the nerve is so small that the 

 total sum of its nervous energy must be very 

 slight. The total energy of this minute machine 

 is so slight that it cannot be detected by our 

 comparatively rough instruments of measure- 

 ment. 



In short, all evidence goes to show that the 

 nerve impulse is a form of motion, and hence of 

 energy, correlated with other forms of physical 

 energy. The nerve is, however, a very delicate 

 machine, and its total amount of energy is very 

 small. A tiny watch is a more delicate machine 

 than a water-wheel, and its actions are more de- 

 pendent upon the accuracy of its adjustment. 

 The water-wheel may be made very coarse and 

 yet be perfectly efficacious, while the watch must 

 be fashioned with extreme delicacy. Yet the 

 water-wheel transforms vastly more energy than 

 the watch. It may drive the many machines in 

 a factory, while the watch can do no more than 

 move itself. But who can doubt that the watch, 

 as well as the water-wheel, is governed by the 

 law of the correlation of forces ? So the nervous 

 system of the living machine is delicately adjusted 

 and easily put out of order, and its action in- 

 volves only a small amount of energy ; but it is 

 just as truly subject to the law of the conserva- 

 tion of energy as is the more massive muscle. 



Sensations. Pursuing this subject further, we 

 next notice that it is possible to trace a connec- 

 tion between physical energy and sensations. Sen- 



