928 CUTANEOUS SENSATIONS. 



excitability is altered to more than unity, the " pain threshold " lying 

 lower than the " touch threshold." 



Many other conditions affecting the skin affect the liminal stimulus. 

 Cold blunts the sensibility at " touch spots," but less than at " pain 

 spots." To stretch the skin much lowers its sensitivity at " touch (and 

 pain) spots," deformation of the skin surface is then, of course, less 

 facile. By rubbing and by scratching, it is easy to raise the liminal 

 stimulus for a local region threefold. Hence the rubbing or scratching 

 to " remove irritation." By practice and attention the liminal stimulus 

 for a patch of skin may be soon reduced to half its original value. 1 



As to drugs, cocain 2 locally applied (5 per cent, solution injected 

 subcutaneously), depresses all forms of cutaneous sensitivity, after caus- 

 ing evanescent hyperalgesia. Tactual sensitivity is more depressed than 

 painful, and less than thermal. Applied to the conjunctiva, cocain 

 abolishes pain more speedily than tactual and than cold sensation. 

 Carbolic acid (5 per cent.) acts similarly to, but less strongly than, 

 cocain. In subcutaneous injections of 2 '5 per cent, solution, it can 

 locally completely paralyse all cutaneous sensation. Chloroform, 

 applied with a sponge, after a temporary burning sensation, leaves 

 sensibility, especially to temperature, blunted for a longer period. 3 

 Orthoform acts as does cocain, but its action lasts longer ; unlike cocain, 

 it is not toxic. 4 Menthol, after producing a temporary hypersesthesia, 

 especially of the " cold spots," depresses sensibility, especially for cold. 

 The feeling of local coolness, caused by applying menthol to the skin, is 

 due less to evaporation than to hyperaesthesia of the end-organs for cold. 5 



The liminal difference of intensity. That smallest increment or 

 decrement of stimulus which occasions a noticeably different degree of 

 sensation can serve as criterion of fineness of gradation of a sense. 

 E. H. Weber, 6 in measuring such " differences," found it best to apply 

 the comparable stimuli successively to the same surface of skin. He used 

 the two distal phalanges of two adjacent fingers, which, with the whole 

 arm, lay fully supported on a rigid surface. The time interval between 

 two successive stimuli must be brief, e.g. 5 sees., and regular, in comparing 

 pairs successively. The duration of the stimuli should be equal. No 

 thermal stimulus should accompany the tactile. By measuring the 

 stimuli, giving "just noticeable differences," Weber found the limit of 

 discrimination for tactile stimuli, evoking sensations from the volar skin 

 of ungual phalanx, was reached when the weights used as stimuli were 

 to each other as 29 to 30. But the ratio was not the same for all skin 

 regions, and it differed in different individuals. The ratio for a given 

 region and individual remains approximately constant throughout a 

 considerable range of absolute value of external stimulus. This is the 

 basis of Weber's " law " ; it applies best to stimuli of moderate intensity. 

 With tactile stimuli, as the stimulus decreases to near liminal intensity, 

 the ratio of increment or decrement required for perceptible difference 

 has to be much greater than for stimuli of moderate intensity ; thus in 

 the palm 1*66 grins, and 1 grm. Similarly, in the case of very intense 

 stimuli, the ratio is departed from. With moderate weights, a difference 

 of pressure is perceptible when the ratio of alteration is smaller than 



1 v. Frey, loc. cit. 



2 Goldscheider, Monatsh. f. prakt. Dermat., Hamburg u. Leipzig, 1886, No. 2, Bd. v. 



3 Goldscheider, loc. cit. 4 Hirschbruch, Semaine intd., Paris, 1897, p. 476. 



5 Goldscheider, loc. cit. 



6 Wagner's " Handworterbuch," Braunschweig, 1846, Bd. iii. Abth. 2, S. 511, etc. 



