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TEXT-BOOK OF PHYSIOLOGY 



The discriminative sensibility of any portion of the body is a function of 

 its mobility. This is shown by the fact that it increases rapidly from the 

 shoulders to the fingers and from the hips to the toes. 



The Temperature Sense. The sensations of heat and cold which are 

 experienced from time to time are caused by changes in the temperature of 

 the skin produced in a variety of ways. As these sensations are specifically 

 different from those of touch, as well as different from each other, it is highly 

 probable that for each sensation there are special nerve-endings distributed 

 throughout the skin. Investigations have shown that all over the skin there 

 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 



FIG. 280. 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.) 



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

 280.) 



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

 by the fact that even o.o5C. is readily appreciable. This holds true, 

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

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

 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 



