1426 ON CUTANEOUS AND [BOOK m. 



senses and indeed to nerves in general. This doctrine teaches that, 

 owing either to the constitution of the central ending of a sensory 

 fibre or to that combined with the nature of the fibre itself (the 

 view may also be adapted to motor fibres), whatever impulses 

 are generated in the fibre can give rise to those events only 

 which are specific to that central ending, impulses of all kinds 

 along an optic fibre giving rise to visual sensations, impulses of 

 all kinds along an auditory fibre giving rise to auditory sensations, 

 and so on. Hence under this view the purpose of the specific 

 terminal organ is simply to allow the specific stimulus of the 

 sense, light in the case of the retina, to develop impulses in the 

 specific nerve, a result which, in the absence of the terminal organ, 

 it is powerless to achieve. 



We saw however ( 750) that according to some observers 

 direct stimulation of the optic fibres, as when the nerve is cut, 

 does not produce visual sensations, and therefore does not give rise 

 to visual impulses ; so far as can be ascertained such a stimulation 

 of the fibres appears to produce no effect at all on the central 

 nervous system. If we accept this result as true, we must modify 

 the doctrine of the specific energy of nerves in the following way. 

 We must suppose that the visual centres are so constituted that 

 they are stirred up to the development of visual sensations by the 

 advent only of those kind of impulses which are started by means 

 of the terminal organ. Since electric changes are developed in 

 the optic fibres as in other nerve fibres when the optic fibres are 

 directly stimulated, we may infer that direct stimulation does lead 

 to nervous impulses ; and we may further infer that these reach 

 the visual centres but are unable to develop visual sensations 

 because they are not true visual impulses such as are generated 

 by help of the terminal organs. 



This modified view is supported, though the support is of a 

 negative kind only, by the behaviour of the other organs of special 

 sense. We have no satisfactory experimental or other evidence 

 that stimulation of the auditory fibres or of the olfactory fibres 

 otherwise than through the terminal organs will give rise to 

 auditory or olfactory sensations. We have evidence that stimula- 

 tion of the centres by various means will give rise to the specific 

 sensations, but not that stimulation of the fibres of the nerves 

 themselves will. The branches of the glosso-pharyngeal and fifth 

 nerves distributed to the organs of taste are, unlike the above, 

 mixed nerves, and when they are stimulated sensations other than 

 specific taste sensations are also developed, and the former might 

 obscure the latter ; still the evidence so far as it goes supports the 

 view that stimulation of gustatory fibres otherwise than through 

 their terminal organs does not lead to the development of gusta- 

 tory sensations. 



In the case of touch the evidence is perhaps still stronger. 

 We must in any case suppose that each cutaneous nerve distributed 



