THE ADEQUATE STIMULUS FOR HEAT AND COLD. 953 



the same time held in water at 15° C, and if then both are placed in water 

 at 40° C, the finger of A feels the warmth better than that of B. These 

 and similar observations show that each temperature stimulus of above a 

 certain intensity involves, besides its action as a stimulus, other consequences 

 for the temperature end-organs. First, it excites end-organs (and their nervous 

 apparatus) for which it is adequate, and therefore involves, especially in view 

 of the long-drawn character of the after sensation, a temporary reduction of the 

 excitability of the apparatus for a second stimulus : the sense organs of tempera- 

 ture are peculiarly easily tired. Secondly, it alters the temperature of the 

 skin in a direction which tends to lessen the difference of temperature between 

 the physical stimulus and the recipient organs, and therefore to reduce the 

 intensity as a stimulus of any repetition of the original or a similar kind of 

 stimulus. Thirdly, by altering the temperature of the skin, if the alteration 

 be carried beyond a small range close above and below the normal skin tem- 

 perature, the excitability of both the cold and warmth organs is markedly 

 lessened by the alteration. 



An experiment laid stress upon as instancing " adaptation " and "contrast " 

 is the following : — A mercury bath is warmed to such a temperature as to feel to 

 the finger neither warm nor cold. From it the finger is transferred to mercury 

 at the temperature of the room. After half a minute or so, the finger, on being 

 transferred to the bath at skin temperature, then "feels it warm." Similarly, 

 if the finger of one hand be held in water at 15° C, and the corresponding 

 finger of the other in water at 35° C, and then both are transferred to water at 

 25° C, the same water is felt warm by one finger, cold by the other. It must 

 be remembered that Hering wrote prior to recognition of the separateness of 

 the physiological apparatus for cold and for warmth sensations. I do not see 

 that the above presents difficulty for Weber's view if the mechanism be postu- 

 lated to be double, as may now be done. Weber's view, equally with Hering's, 

 accounts for bodies cooler than the skin giving rise to "warmth" sensation. But 

 there are experiments of Weber's own which seem to me to argue strongly 

 against his view. If, for instance, at the ordinary temperature of the room, one 

 presses a cold piece of metal (2° to 8° C.) for 20 seconds against the palm, the 

 temperature of the skin there falls 5° to 8° C. The sensation is painful. After 

 removal of the metal, the cooled patch of skin gets warmer again, rapidly at 

 first, later more slowly. Even in eight minutes' time it does not fully reach its 

 previous temperature, although its temperature is rising all the time. Through- 

 out the whole of that period, although the objective temperature of the skin is 

 rising, a cold sensation is clearly felt. A converse experiment can be per- 

 formed with warm metal. These results offer difficulty to Weber's view, but 

 can be explained by Hering's. In these cases the periods seem altogether too 

 long to be explicable by positive after sensations, even combined with con- 

 comitant lowering of excitability. 



Again, as we sit before a fire, we can for hours continue to enjoy sensations 

 of warmth from a surface of skin exposed to it. But we cannot suppose that 

 under such circumstances a continual rising of the temperature of the skin and 

 its thermal organs is lastingly going on. And, conversely, long-continued sensa- 

 tions of cold, produced under conditions that do not vary, are facts of experience 

 which Weber's theory of the adequate stimulus for the thermal sense fails to 

 meet successfully. 



Hering's appears the most probable theory. It seems to me altered 

 by the recent dual analysis of the thermal sense only in so far that we 

 must suppose that the difference between the temperature of the 

 end-organ and its physiological zero temperature (settled by adaptation) 

 — which difference is, in his view, the cause of sensation — acts, when the 

 existing temperature is higher, only on the one set of end-organs (apart 

 from paradoxical sensation) ; when the existing temperature is lower, only 



