316 BIOLOGY 



temperature when a nerve impulse passes over a nerve. This 

 is not, however, an objection to the general theory, since the 

 nerve is such a small machine that it would be doubtful whether 

 our tests are delicate enough to recognize any rise in tempera- 

 ture even if such a rise occurred. The total energy of the 

 nervous impulse is too small to be detected by our rough 

 instruments for measuring heat. 



All evidence goes to show that the nervous impulse is a 

 form of motion, and hence is correlated with other forms of 

 physical energy. The nerve is a very delicate machine and its 

 total amount of energy is very small. A tiny watch is more 

 delicate than a water-wheel, and its actions are more closely 

 dependent upon the accuracy of its adjustment. The water- 

 wheel may be made very coarsely and still be useful, while the 

 watch must be fashioned with extreme care and nicety. Yet 

 the water-wheel transforms vastly more energy than the watch ; 

 it may drive the machinery of the whole factory, while the 

 watch can 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 nerve machine of the living 

 body is delicately adjusted, easily put out of order, and its 

 actions involve only a small amount of energy; but it is prob- 

 ably just as truly subject to the law of the conservation of 

 energy as are the more massive muscles. 



Sensations. Up to a certain point, sensations can also be 

 related to the general problem of the conservation of energy. 

 The frog has a piece of apparatus, which we call the ear, capable 

 of being affected by the vibrating waves of the air. It is made 

 of parts so delicately adjusted that the air waves set them in 

 motion, and this motion starts a nervous stimulus which travels 

 along the auditory nerve to the brain. Whenever air waves 

 strike the frog's ear, they will excite in his auditory nerve 

 impulses which will travel from the ear to the brain. The ear 

 is simply a delicately poised apparatus, so adjusted that when 

 it is stimulated by vibrating air it is discharged like a bit of 



