IS THE BODY A MACHINE? 59 



sations are excited by certain external forms of 

 motion. 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 

 delicately 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 whenever that external type of motion 

 which we call sound strikes the ear. In other 

 words, the ear is a piece of apparatus for chang- 

 ing air vibrations into nervous stimulation, and 

 is therefore a machine. 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 develops any energy other 

 than that in it at the outset. In the same way 

 the optic nerve has, as its end, a bit of mechan- 

 ism 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 optic 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 energy, just as a 

 dynamo is a machine for transforming motion 

 into electricity. If the machine is broken, the 

 external force has no longer any power of acting 

 upon it, and the individual becomes deaf or blind. 

 Mental Phenomena. Thus far in our analysis 



