XXXV.] NERVES OF SPECIAL SENSATIUX. 1"5 



trunk of either of these nerves gives rise to pain, which is nut the 

 case with either the olfactory, the optic, or the auditory nerves. 

 Moreover, the papUlary apparatus, through which the gustative 

 impressions are made upon the extremities of these nerves, is 

 essentially the same in structure with that of the skin." ^ 



jMoreover, if there is a distinct set of visual nerve-fibres for 

 each primary colour, it would appear to follow that there must 

 be a distinct set of nerve-fibres of taste for each distinct kind 

 of taste — saline, bitter, sweet, &<:.; and a distinct set of olfactory 

 nerve-fibres for each distinct kind of smell ; which would be 

 an almost iucretUble conclusion. The case of the sense of 

 hearing, as we have seen in the preceding chapter, is different 

 from these. 



Each distinct sound is probably transmitted to the auditory 

 ganglia by a distinct set of nei'ves. Eut, as a result of this, it 

 is possible to distinguish, as distinct, sounds which are received 

 in the consciousness together ; and if the impressions of different 

 2)riniary colours were transmitted to the optic ganglia by different 

 nerve-fibres, we should be able to distinguish the constituent 

 colours in a compound colour — we should be able to see the 

 three primaries in white. 



If it were true that any one nerve- fibre can transmit sensations 

 of only one kind, it must follow that the nerves of the sensa- nor of 

 tion of heat must be distinct from those of touch ; but, accord- 

 ing to Dr. Carpenter, there is no evidence whatever in favour of 

 this conclusion.^ 



The truth appears to be, that the sensations which any nei've 



transmits depend neither on the constitution of the nerve itself, Thekiud 



nor on that of its ganglion (for all ner\-es and all ganglia, so lar "f seiisa- 



as the microscope has shown, are histologically alike), but alto- pends nut 



gether on the constitution of the organ in which the nerve has ^'^ ^^® 



. ^ nerves uor 



its external termination. Thus a nerve which terminates in the outhegau- 



eve transmits the sensation of light, and a nerve which termi- S^^\ ^^^'^^ 



•' ° T ., on the 



nates in the ear transmits the sensation of sound. The difier- organ of 



ences in kind between the sensations of different colours are no sense. 

 doubt left uuexplaiued by this liypothesis, but they are inex- 

 plicable by any hypothesis whatever, and the difierence between 

 the tastes of sugar and of salt, or any other simple fact of 

 sensation, is equally inexplicable. 



The conclusion stated in the foregoing paragraph may appear 



' lluiuaii riiysinldgy, ]i. (il7. " Ibid. |i. tjlT). 



