i CUTANEOUS SENSIBILITY 5 



must add to the cutaneous sense a pressure sense, a temperature 

 sense, and a pain sense; lastly, according to the quality of 

 sensation, the thermal sense must be subdivided into a heat sense 

 and a cold sense. The psychological classification, founded on 

 the dissimilar nature of the sensations, is evidently the most 

 analytical and, therefore, the most rational to employ in defining 

 and distinguishing the sense-organs. 



It is important to notice that two kinds of dissimilarity can 

 be distinguished in the comparative study of sensations. Helm- 

 holtz (1879) made a distinction between differences in modality 

 and simple differences of quality. Sensations of different modality 

 are so fundamentally dissimilar that transition from one to the 

 other is not possible ; no degree of similarity, nor even a simple 

 relation of intensity, can be established between them. No one, 

 for instance, can say whether a given musical tone resembles more 

 closely the colour red, or a bitter taste, or the scent of musk ; nor 

 decide whether the light of a candle is stronger or weaker than 

 the sensation evoked by a certain solution of sugar, a given 

 musical note, a sensation of pressure or temperature in the skin. 

 If, on the other hand, we compare the sensations appreciable within 

 each modality, we can indeed recognise qualitative differences ; but 

 these are not so profound as to make impossible a reciprocal 

 transition from one to the other, or a comparison and judgment 

 of their greater or less similarity, greater or less intensity. Two 

 separate auditory sensations may be qualitatively distinguished 

 by their difference of pitch ; it is also possible to judge which of 

 them is the stronger. The colours of the spectrum not only present a 

 gradual transition from one to the other, but we can also appreciate 

 their greater or less resemblance or their relative brightness. 



The differences between the modalities of sensation observed 

 on examining the higher sense-organs of vision and hearing, both 

 in their mutual relations and in the relations between each of 

 them and the lower sense-organs, could not well be more profound 

 and striking. But this conspicuous disparity does not appear on 

 comparing the sensations that arise from the less well-developed 

 sense-organs. 



Tick (1879) first pointed out that the sensations of smell, 

 taste, touch, temperature, and pain are modalities not so different 

 in themselves that a gradual transition from one to the other is 

 impossible. Thus, between the sensation of pricking produced by 

 pepper on the tongue and that produced by a solution of table 

 salt, the former being a tactile and the latter a gustatory sensation, 

 a gradual transition is possible by means of a series of salt 

 solutions and pepper extracts of increasing strength. In this 

 case, therefore, the difference in modality assumes the character 

 of differences in quality, between which a gradual transition is 

 possible, as between the colours of the solar spectrum. 



