636 



TEXT-BOOK OF PHYSIOLOGY. 



are innumerable spots of varying size which if stimulated evoke sensations of 

 heat or cold. Such points are termed heat and cold spots. Each responds 

 to but one kind of stimulus. A warm object applied to a heat spot will 

 evoke a sensation of warmth. It will have no effect on the cold spot. The 

 reverse is also true. Between the cold and heat spots there are areas that 

 are neutral, insensitive to either heat or cold. The cold spots are more 

 numerous than the heat spots in almost all regions of the body. (See Fig. 

 298.) 



The sensitivity of the skin to temperature changes is very acute, as shown 

 by the fact that even 0.05 C. is readily appreciable. This holds true, 

 however, only when the temperature of the object lies between 27 and 33 C. 

 This capability varies in different regions of the skin, and depends on the 



FIG. 298. COLD AND HOT SPOTS FROM THE ANTERIOR SURFACE OF THE FOREARM, a. Cold 

 spots, b. Hot spots. The dark parts are the most sensitive, the hatched the medium, the dotted 

 the feebly, and the vacant spaces the non-sensitive. (Landois and Stirling.) 



number of heat and cold spots present, the thickness of the epidermis, the 

 thermal conductivity of the object touching it, and the extent to which it is 

 habitually exposed or protected. 



The physiologic stimulus to the thermic end-organs is the passage of 

 heat through the skin from the interior of the body to the surrounding air. 

 If the radiation is continuous and uniform, the end-organs soon adapt them- 

 selves to the temperature of the surrounding air and the sensation of heat, 

 under physiologic conditions, is not evoked. If there is a sudden rise in the 

 external temperature caused by natural or artificial means, which diminishes 

 the radiation, the temperature of the skin will at once rise, the end-organs 

 will be stimulated, and a sensation of warmth developed. If, on the other 

 hand, there is a sudden fall in temperature and an increased radiation, the 

 temperature of the skin will fall, the end-organs will be stimulated, and a 

 sensation of cold developed. Experiment also teaches that the intensity of 

 a warm or cold sensation will depend on the existing temperature of the skin, 

 and not upon the absolute temperature of the object. Thus, water at 20 C. 



