SPECIFIC NERVE ENERGIES. 191 



would be to expose directly a very delicate end branch of 

 the axis cylinder, and this seems in fact to be the case in 

 the nerves of the transparent exposed part of the eyeball, if 

 not in many other parts of the external integument of the 

 Body. But as axis cylinders are quite unirritable by light 

 or sound a mere exposure of them would be useless in the 

 eye or ear, and in each case we find accordingly a very 

 complex apparatus developed, whose function it is to 

 convert these modes of motion which do not excite nerves 

 into others which do. We might compare it to a 

 cartridge, which contains not only "irritable" gunpowder 

 but highly "irritable" detonating powder, and the stimulus 

 of the blow from the hammer which would not itself ignite 

 the gunpowder, acting on the detonating powder (which 

 represents an "end organ"), causes it to give off a spark 

 which in turn excites the gunpowder, which answers to the 

 nerve-fibre. 



Specific Nerve Energies. We have already seen that a 

 nervous impulse propagated along a nerve-fibre will give 

 rise to different results according as different nerve-fibres 

 are concerned, Traveling along one fibre it will arouse a 

 sensation, along another a movement, along a third a se- 

 cretion. In addition we may observe readily that these 

 different results may be produced by the same physical 

 force when it acts upon different nerves. Radiant energy, 

 for example, falling into the eye causes a sensation of 

 sight, but falling upon the skin one of heat, if any. The 

 different results which follow the stimulation of different 

 nerves do not then depend upon differences in the physical 

 forces exciting them. This is still further shown by the 

 fact that different physical forces acting upon the same 

 nerve arouse the same kind of sensation. Light reaching 

 the eye causes a sight sensation, but if the optic nerve be 

 irritated by a blow on the eyeball a sensation of light is 

 felt just as if actual light had stimulated the nerve ends. 

 So too when the optic nerve is cut by the surgeon in ex- 

 tirpating the eyeball, the patient experiences the sensation 

 of a flash of light; and the same result follows if an electric 



