32 PHYSIOLOGY CHAP. 



in the skin. So long as the temperature of any part of the skin 

 remains constant between certain mean limits, there is no ex- 

 citation ; but as soon as the temperature of this region changes, 

 either from external or internal reasons, thermal sensations at 

 once arise. 



Normally a slow, continuous thermal current flows through 

 the skin from within, outwards. So long as the conditions of this 

 current remain unchanged, the temperature of the nerve-organs 

 remains the same ; but if the current alters with a certain 

 rapidity, there is a sensation of warmth or cold in consequence of 

 the rise or fall of temperature in the end-organs. According to 

 Weber it is these changes in the temperature of the end-organs 

 which constitute the adequate stimulus and the essential conditions 

 of thermal sensation, no matter what caused the alteration of 

 temperature. It almost seems, he writes, as if we could detect the 

 process of rise and fall in the temperature of our skin much better 

 than the degree to which the temperature rises and falls. Since 

 the discovery of specific organs for cold and heat, it has become 

 possible to give a more exact definition to Weber's theory, by 

 saying that the organs for cold are excited by fall of their 

 temperature, and those for heat by its rise. 



This theory gives a satisfactory explanation of many facts. 

 We are aware of a sensation of cold both when the loss of heat 

 through the skin increases, and when the peripheral blood-supply 

 diminishes. We have a sensation of warmth both when the loss 

 of heat by the skin is decreased in consequence of a rise in the 

 temperature of the environment, and when the peripheral blood- 

 supply increases. Accordingly, it is not the direction of the 

 thermal heat current from within outwards, or from without 

 inwards, nor the intensity of this current, which produces the 

 thermal sensations, as assumed by Vierordt, but the changes in 

 temperature at the thermal end-organ, no matter what process 

 causes them. 



One fact, however, seems at first sight to contradict Weber V 

 theory. If a metal at 3 C. is applied for some time to any part 

 of the skin, for instance the forehead, and then removed, there 

 will for some 20 seconds be a sensation of cold in that part 

 instead of heat, as would be the case if the skin were growing 

 warmer. Fechner and Vierordt also noted that it is possible to 

 feel a prolonged sensation of heat or cold without any change in 

 the temperature of the environment. These facts led Hering to 

 conclude that not only thermal changes, but the absolute degree 

 of cutaneous temperature as well, may act as a stimulus of 

 thermal end-organs. 



When the temperature of the environment remains fairly 

 constant we are not as a rule aware of any thermal sensation, 

 although the different parts of the skin may have a very different 



