L052 ON CUTANEOUS AND [Book in. 



to a visual sensation and must therefore be regarded as a visual 

 impulse. This view, under the title of the doctrine of " the spe- 

 cific energy of nerves," has been extended to the nerves of the 

 other special senses and indeed to nerves in general. This doc- 

 trine 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 audi- 

 tory 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 develope 

 impulses in the specific nerve, a result which, in the absence of 

 the terminal organ, it is powerless to achieve. We saw however 

 (§ 553) 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. 

 In arguing from this result we must remember that the optic nerve 

 is not a true nerve and that its fibres are not comparable with the 

 fibres of a true nerve ; the optic nerve is a part of the brain, and 

 its fibres are analogous to the internuncial fibres of the white mat- 

 ter of the central nervous system, concerning which as we saw 

 (§ 510) the view has been urged that they are not capable of 

 being stimulated directly. Neglecting however this view which 

 is at best very doubtful, we are led, if we accept the insensitive- 

 ness of the optic nerve to direct stimulation as true, to 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 develope visual sensations 

 because they are not true visual impulses such as are generated 

 by help of the terminal organs. 



The facts which we mentioned in speaking of hearing (§ 628) 

 as seeming to shew that the fibres of the auditory nerve in the 

 absence of the labyrinth may be directly stimulated by sound, if 

 we accept them as valid, afford a strong support to the simpler 

 conception of the specific energy of nerve fibres. On the other 

 hand the modified view is supported, though the support is of 

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



