LIMINAL DIFFERENCES FOR THERMAL SENSITIVITY. 959 



moment be producing any thermal sensation, finger and ear being each fully 

 adapted to their physiological zero temperatures, on bringing the finger in 

 contact with the ear, the finger may feel the ear to be cool, the ear feel the 

 finger to be warm. 



Chiefly among the great group of intactual unprotected thermal sense 

 reactions lie those which reflexly help to regulate the general temperature of 

 the homoiothermous animal. These reactions can scarcely be considered to 

 come within the scope of this chapter so fitly as under that of animal warmth. 

 Although reactions of which the initiation is due to the operation of the 

 afferent mechanism of the specific senses of " cold " and " warmth," it seems 

 certain that some of these reflexes do not directly influence the stream of 

 consciousness. They are unconsciously executed, except when unusually 

 intense. When sufficiently intense to affect consciousness, they are especially 

 liable to assume pleasurable or painful characters of "feeling." 



The liminal difference of intensity of stimulus. — Starting from 

 the temperature sensation evoked by an adequate stimulus of threshold 

 intensity, a very considerable range of intensities of sensation can be 

 distinguished by increments of stimuli, and that without otherwise pro- 

 ducing any change in the quality of sensation. Between certain limits 

 of temperature the values of the threshold differences are very small. 

 Above, and especially below, those limits the values of the threshold 

 differences become much greater. Fechner, by applying successively to 

 the same area of skin two objects different in temperature but otherwise 

 similar, estimated the smallest difference of temperature (of water) 

 perceptible by the thermal organs of the skin at the area in question. 

 He found the minimal perceptible difference smallest with stimuli 

 between the temperatures of 12°*5 C. and 25° C. Using water and the 

 skin of the finger, he discovered that when the finger had been allowed 

 sufficient time in the first specimen of water for, as would now be said, 

 some " adaptation " of its end-organs to the temperature of that water, 

 it could detect differences of temperature to a degree which did not 

 affect his thermometer, although graduated in twentieths of Eeaumur 

 units. The range of temperature (12 0, 5 C. to 25° C.) between which 

 Fechner found the liminal difference smallest must have applied 

 entirely to " cold " sensation, since the skin of the hand possesses 

 normally a temperature ranging between 27° C. and 29 3 C. 1 Alsberg, 2 

 however, found the threshold difference smallest with stimuli (water) 

 between the temperatures of 35° C. and 39° C, a range which, as the 

 highest skin temperature is about 32° C., 3 must have applied to 

 " warmth sensation." It might have been expected that the ran we 

 of temperatures at which the threshold difference was smallest would 

 have lain close above and below the " adequate temperature." Some 

 observers 4 have indeed obtained this result, notably Nothnagel, who 

 found the range lie between 27° C. and 33° C. 



As with tactual intensity, the threshold difference between two 

 thermal stimuli applied successively to the same area of skin is much 

 less than between stimuli simultaneously applied to similar but not 

 identical skin areas, even if they lie near together. 5 



Hensen, 6 to whom is owing the introduction of the hair-aesthesiometer for 



1 Kurtkel, Sitzungsb. d. phys.-med. Gescllsch. zu Wiirzburg, 1886, S. 79. 



2 E.g. Lindemann, "Diss.," Marburg, 1863. 3 Kunkel, loc. cit. 



4 Deutsches Arch. f. klin. Med., Leipzig, Bd. ii. S. 284. 



5 Weber, loc. cit. 6 Arch./. Ohrenh., 1893, Bd. xxxv. S. 161. 



