46 THE STORY OF THE LIVING MACHINE. 



a small amount of energy ; but it is just as truly 

 subject to the law of the conservation of energy 

 as is the more massive muscle. 



Sensations. Pursuing this subject further, we 

 next notice that it is possible to trace a connection 

 between physical energy and sensations. Sensa- 

 tions are excited by certain external forms of mo- 

 tion. The living machine has, for example, one 

 piece of apparatus capable of being affected by 

 rapidly vibrating waves of air. This bit of the 

 machine we call the ear. It is made of parts deli- 

 cately adjusted, so that vibrating waves of air set 

 them in motion, and their motion starts a nervous 

 stimulus travelling along the auditory nerve. As 

 a result this apparatus will be set in motion, and 

 an impulse sent along the auditory nerve when- 

 ever that external type of motion which we call 

 sound strikes the ear. In other words, the ear is 

 a piece of apparatus for changing air vibrations 

 into nervous stimulation, and is therefore a ma- 

 chine. Apparently the material in the ear is like 

 a bit of gunpowder, capable of being exploded by 

 certain kinds of external excitation ; but neither 

 the gunpowder nor the material in the ear de- 

 velops any energy other than that in it at the out- 

 set. In the same way the optic nerve has, at its 

 end, a bit of mechanism readily excited by light 

 vibrations of the ether, and hence the optic nerve 

 will always be excited when ether vibrations 

 chance to have an opportunity of setting the op- 

 tic machinery in motion. And so on with the other 

 senses. Each sensory nerve has, at its end, a bit 

 of machinery designed for the transformation of 

 certain kinds of external energy into nervous en- 

 ergy, just as a dynamo is a machine for trans- 

 forming motion into electricity. If the machine is 



