244 THE COMMON SENSES 



Tactile areas have, in general, an oval form, with the long 

 axis parallel to the long axis of the portion of the body investi- 

 gated. These areas are not indicative of the distribution of 

 certain nerves. The important factor in the separation of two 

 points that are stimulated is not that two different nerves shall 

 be stimulated, but that there must be a certain number of 

 unstimulated points between those stimulated. 



The sense of touch can be greatly educated and specialized. 

 The reading of raised letters by the blind is an example. Im- 

 proved touch discrimination, attained by practice upon the 

 skin of one arm, is accompanied by an improvement in the 

 corresponding area of the other arm, but not of any other areas 

 of the body. This shows that the localizing power lies within 

 the central system. 



The skin is an organ for the detection of temperature changes, 

 and its power in this respect varies in different portions of the 

 body. The intensity of the sensation depends upon the area 

 stimulated. There is very little doubt that there are two 

 distinct temperature nerves, which serve respectively for the 

 appreciation of heat and cold. The areas to which the nerves 

 are distributed can be located in the skin as cold and heat points. 

 Temperature sensations are not accurate; they are only relative 

 that is, the temperature of various things is inferred from the 

 temperature of the skin and its habitual surroundings. It is 

 related that Arctic explorers have found the water warm when 

 swimming in pools on icebergs, and a drop of mercury at 

 80 F. is said to feel cold in the tropics. A more simple illus- 

 tration is that of immersing one hand in water at 40 F. and 

 the other in water at 120 F., and then plunging both into 

 water at 80 F., when one hand will feel hot and the other cold. 

 During a chill the temperature of the body is often very high, 

 and yet the sensation is that of cold. 



Common Sensation. By common sensation is meant that 

 state of mind, more or less definite, by which the condition 

 and position of the body at any moment are known. Such 

 perceptions cannot be located distinctly in any organ or set 

 of organs, as, for instance, hunger, thirst, etc. Besides these 

 there are some sensations which involve certain organs which 

 must be classed under this head; thus, inclinations to cough 

 or to sneeze, to vomit, defecate, and urinate. Many of these 



