GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 197 



liquids. (6) Mechanical stimuli when so feeble as to be ineffi- 

 cient as general stimuli. Pressure on the skin of the fore- 

 head or of the back of the hand, equal to .002 gram (.03 grain) 

 can be felt through the end-organs of the sensory fibres there, 

 but would be quite incapable of acting as a general stimulus 

 if applied directly to the nerve-fibre. 



It will be noticed as regards the special stimuli of afferent 

 nerves that many of them are merely less degrees of general 

 stimuli ; the end-organs in skin, mouth, and nose are in fact 

 excited by the same things as nerve-fibres, but they are far 

 more irritable. In the case of the higher senses, seeing and 

 hearing, however, the end-organs seem to differ entirely in 

 property from nerve-fibres, being excited by sonorous and 

 luminous vibrations which, so far as we know, will in no 

 degree of intensity directly excite nerve-fibres. To construct 

 an end -organ capable of recognizing very slight pressures we 

 may imagine that all that would be needed would be to expose 

 directly a very delicate end-branch of the axis cylinder ; and 

 tnis seems in fact to be the case in the nerves of the-transpar- 

 ent exposed part of the eyeball, if not in some 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 modes of motion which do not 

 excite nerves into others which do. We might compare 

 this apparatus to a fuse with a detonating cap attached ; the 

 stimulus of a blow from a hammer which would not itself 

 ignite the fuse, acting on the detonating material (repre- 

 senting an " end -organ"), causes it to give off a spark, and 

 this in turn ignites the fuse 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. Travelling along one fibre it will arouse a sensa- 

 tion, along another a movement, along a third a secretion. 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 



