THE SENSE OF TOUCH. 



insensitive to either heat or cold. The cold spots are more numerous 

 than the heat spots in almost all regions of the body<0K 



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 2 7 and 33 C. 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 



FIG. 273. 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. 



of heat through the skin from the interior of the body to the sur- 

 rounding air. If the radiation is continuous and uniform, the end- 

 organs soon adapt themselves 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 wall 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 radia- 

 tion, 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 



