THE SKIN 393 



The skin in the median line of the body is 

 comparatively insensitive to temperature varia- 

 tions. 



2. Similarly compare the mucous membrane 

 with the skin. 



The mucous membranes are much less sensitive 

 than the skin. 



SENSATIONS OF PRESSURE 



Pressure Spots. Explore the surface of the 

 forearm by bringing the blunted point of a needle 

 gently in touch with the skin. 



At certain spots a distinct sensation of contact 

 will be perceived. Other spots will give only 

 dull sensations. Pressure, like heat and cold, is 

 appreciated by scattered sense-organs in the skin, 

 not by diffuse general sensation. 



Note the relation of the pressure points (1) to 

 the hair follicles, and (2) to the warm and cold 

 spots mapped out in previous experiments. 



Threshold Value. Take from the human head 

 several straight, strong hairs. Cement each to 

 the end of a little stick of soft pine to serve as 

 a handle. Provide a special lever, made as fol- 

 lows : With a hot pin burn a small hole at the 

 middle of a straw about 25 cm. in length. Pass a 

 needle through this hole into a cork held in the 

 muscle clamp. Press the free end of the hairs 



