THE SENSE OF FEELING. 255 



sation of light ; the current, on the other hand, which is transmitted to the outer 

 extremity of the fibre situated in the globe of the eye, meets there with no ap- 

 paratus by which it can manifest any effect of a phenomenal nature. If, there- 

 fore, we divide the optic nerve near the brain, so as to sever its collective fibres 

 from their termioal apparatus in the central nerve system, all irritation applied 

 between the point of division and the outer extremity remains ineffectual, and no 

 sensation can arise, just as no intelligence can be transmitted to a telegraph 

 station between which and the place of operation the wire has been sundered. 

 Heretofore it was thought that generally a nerve of sensation was capable only 

 of conducting a current in one direction, namely, from its external to its internal 

 extremity, while the nerve of motion, on the other hand, had no conducting 

 capacity except in the opposite direction. At present, we know with certainty 

 that every nerve fibre is alike qualified to conduct in both directions, the earlier 

 and erroneous view having arisen from the circumstance that no means were 

 known for demonstrating that the current may, in a nerve of sensation, be prop- 

 agated towards the outer extremity, and, in a nerve of motion, towards the inner 

 one, and hence the fact was rashly denied. Now that we possess an admirable 

 expedient for rendering the existence of a current perceptible at any point of a 

 nerve, we know that, for example, the optic nerve can conduct a current from 

 the brain to the globe of the eye, though we also know that in life no current is 

 conducted in this direction, because at the central extremity of the nerve there 

 exists no apparatus of excitation; and even were an irritant applied at that 

 extremity the current flowing outwardly must remain without effect, because at 

 the external extremity there exists no apparatus adapted to the manifestation of 

 its activity and the production of the related phenomena. 



If we now ask further, how it happens that excitation of the optic nerves al- 

 ways produces a sensation of light, excitation of the auditory nerves a sensa- 

 tion of sound, and therefore by what causes single nerves ai'e adapted to the 

 transmission of different specific sensations, but one general answer can be 

 given in advance. Formerly a solution was supposed to have been found by 

 ascribing to each different nerve a different specific faculty, which was called 

 its " specific energy," without proof, and without further explanation of the ex- 

 pression. At present, when clearer views have been obtained of the properties 

 and powers of the nerves, a conviction has prevailed that all nerve-fibres pos- 

 sess like essential properties, and therefore like capacities for action. If this be 

 the case, the causes ot the difference in their actual operation should be sought 

 for solely in tlie apparatus on which they operate ; and we must suppose that 

 what occurs in the excited optic nerve is the same with what occurs in the ex- 

 cited auditory nerve, and also in the nerves of motion, but that at the ex- 

 tremity of the first there is an appropriate apparatus which converts the ner- 

 vous current into a sensation of light, at the extremity of the auditory nerve 

 another which produces from the same current a sensation of sound, while the 

 union of other nerves with the muscles enables the current to accomplish the 

 phenomenon of motion. Mankind in general are disposed to believe, without 

 closer inquiry, that the optic nerve transmits its peculiar sensation because it is 

 stimulated by light, as does the nerve of hearing, because it is stimulated by 

 sound, &c. ; but it is easy to prove that the difference in the effect produced is 

 not dependent on the difference in the means of excitation. Electricity is an 

 irritant which stimulates all the nerves, but when we excite the optic nerve by 

 an electric stroke, a sensation of light is invariably the result, as is that of sound 

 when the auditory nerves are similarly assailed, the same agent of excitation 

 thus producing a different effect, and in the case of each nerve the effect proper 

 to it, .which we must therefore regard as being determined by the nature of the 

 apparatus of the inner extremity. If we examine this apparatus in different 

 nerves with a view to detect supposed differences, we at once encounter the 

 hitherto inexplicable riddle before referred to. So far as microscopic examina- 



