962 CUTANEOUS SENSATIONS. 



of body surface was renewed by the " difference " method. The results 

 were confirmatory. The differences varied between ■2° C. and l°'l C. 

 for different regions of skin. 



It would hardly, perhaps, have been expected that regions endowed with 

 very high absolute sensitivity should be poorly endowed with ability to distin- 

 guish difference. Yet a comparison of the performances of the ungual phalanx 

 and nipple shows it to be so. Evidently in testing ability to distinguish, 

 a very complex process is being examined. Absolute sensitivity must, of course, 

 enter into it, but "experience" is probably just as important. The following 

 is an example : — One and the same " cold " stimulus, applied successively to 

 the nipple and the finger, evokes a more intense cold sensation from the 

 former than from the latter, because the "absolute sensitivity" of the nipple 

 is the greater. But with the finger a difference of temperature between two 

 similar objects can be noted, the temperatures of which stand only *2° C. apart, 

 while for detection by the nipple their difference must be '8° C. Speaking 

 generally, the threshold difference is low in value in regions where " touch " 

 possesses strong local sign. As instanced above, it is a fact, elicited by more 

 than one method, that sensitivity to cold and warmth increases as the skin of 

 the limbs is examined in ascending direction from the apex of the limb 

 toward the trunk. 



It might have been supposed that that surface of the body habitually 

 protected by clothes would have possessed higher sensitivity to cold than 

 would parts habitually naked. On the contrary, it is found that the face is 

 more sensitive to cold than are many clothed parts of the trunk and limbs. 

 The clothed parts of the body surface are less practised in changes of 

 temperature than the unclothed ; the appreciation of degree of change of 

 temperature by the clothed parts has been found correspondingly slight. 



Ansemia of the forefinger by holding it above the head diminishes the 

 minimal interval perceptible between two temperatures. 1 Passive hyperemia 

 seems to affect it little. 2 Long application of heat or cold greatly damages the 

 discrimination. 3 



Reaction-time. — On application of a cold object to the skin, the 

 resulting sensation of cold occurs after only a short period of latent 

 excitation. 4 In its short latency it resembles a " touch." On the 

 contrary, the generation of a warmth sensation is a longer process ; the 

 sensation, like' that of pain, develops slowly. 5 When the hand is 

 dipped in cold or hot water, the back of the hand perceives the 

 temperature sooner than does the palm. Weber concludes this is 

 because the palmar epidermis is the thicker. 6 If both hands are 

 plunged into a deep vessel containing water at 3° to 4° C, and the 

 palms lie toward each other without touching, the cold is felt first by 

 the backs of the hands, but after about eight seconds the feeling of 

 cold is greater in the palms than in the backs ; hence from the palms, 

 although a higher maximal cold sensation can be evoked, a longer 

 period of latent excitation is gone through : the difference in all likeli- 

 hood lies in difference of rate of conduction to the end-organs. 



If an object at temperature T be applied to the surface of the epidermis, 

 and 6 be the physical temperature of the thermal end-organs, which are at the 

 time at their physiological zero-temperature, 6 1 can represent the temperature 

 which the end-organs will take after a certain duration of the application. 



1 Alsberg, loc. cit. - Alsberg, loc. cit. 



3 See Nothnagel, loc. cit.; also Goldsch eider, loc. cit. 4 Goldscheider, loc. cit. 



5 Herzen, Arch. f. d. ges. Physiol., Bonn, 1886, Bd. xxxvii. 



B Loc. cit.; see also for the thermo-conductivity of the skin, Klug, Ztschr. f. Biol., 

 1874, Bd. x. S. 73. 



