GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 201 



fact that in no other sense-organ has the direct stimulation 

 of the proper nerve-trunk in any other way than through 

 the sense-organ at its outer end, been proved to give rise 

 to true sensations of special sense. Stimulation of the nerves 

 exposed in a wound does not cause a true touch sensation, 

 but a feeling of pain; and similarly irritation of the trunks 

 of the nerves of taste by diseased conditions does not seem to 

 ever cause true taste sensations unless the end-organs in the 

 mouth be also excited. Even if it turn out to be true that 

 a nerve of special sense is only capable of giving rise to 

 feelings belonging to the sphere of that sense when ex- 

 cited through its proper end-organs, that does not prove 

 that its nerve-fibres have any unique faculty distinguishing 

 them in property from other nerve-fibres. It only means 

 that the brain organ, the central nerve-cell mechanism, to be 

 excited by the nerve is highly complex, and only responds 

 with the proper sensation when stimulated in proper strength 

 and proper rhythm, and the sense organs accomplish this. 

 Even the most delicate artificial stimulation that we can 

 apply to a naked nerve-trunk is undoubtedly a crude and 

 gross thing compared with the stimuli arising in the retina 

 when light enters the eye, or in certain skin nerve end- 

 organs when moderate heat falls on them. If we merely 

 wish to get a noise out of a piano it does not matter how 

 we strike it, if we strike hard enough ; and a muscular con- 

 traction or an irregular set of muscular contractions excited 

 by direct stimulation of a nerve-trunk may be compared to 

 such a noise. If we wish for a definite musical chord we 

 must strike through the keyboard in a definite way; and the 

 orderly combined muscular movements and the special sensa- 

 tions which follow stimulation through the proper sense- 

 organs may be compared to such chords. In our bodies the 

 keyboards are different in eye, ear, and skin, and adapted to 

 be set in action by different external physical agencies, and 

 the strings in connection with each keyboard are different 

 and give different results; but the connecting apparatus, the 

 nerve-fibre, lying between the keys in the sense-organs and 

 the strings respectively responding to them in the centres, 

 is essentially the same in all cases. 



To put the case more definitely: Light outside the eye 

 exists as ethereal vibrations, sound outside the ear as vibra- 

 tions of the air (commonly). Each kind of vibration acts on 



