182 THE HUMAN BODY 



time. Whenever a skin region passes with a certain rapidity to 

 a temperature above its sensation zero we feel warmth; and vice 

 versa: the sensation is more marked the greater the difference, 

 and the more suddenly it is produced; touching a metallic body, 

 which conducts heat rapidly to or from the skin, causes a more 

 marked hot or cold sensation than touching a worse conductor, 

 as a piece of wood, of the same temperature. 



The change of temperature in the organ may be brought about 

 by changes in the circulatory apparatus (more blood flowing 

 through the skin warms it and less leads to its cooling), or by 

 temperature changes in gases, liquids, or solids in contact with it. 

 Sometimes we fail to distinguish clearly whether the cause is 

 external or internal; a person coming in from a windy walk often 

 feels a room uncomfortably warm which is not really so; the 

 exercise has accelerated his circulation and tended to warm his 

 skin, but the moving outer air has rapidly conducted off the extra 

 heat; on entering the house the stationary air there does this less 

 quickly, the skin becomes hotter, and the cause is supposed to be 

 oppressive heat of the room. Hence, frequently, opening of win- 

 dows and sitting in a draught, with its concomitant risks; whereas 

 keeping quiet for five or ten minutes, until the circulation had 

 returned to its normal rate, would attain the same end without 

 danger. 



The acuteness of the temperature sense is greatest at tem- 

 peratures within a few degrees of 30 C. (86 F.) ; at these differ- 

 ences of less than 0.1 C. can be discriminated. As a means of 

 measuring absolute temperature, however, the skin is very un- 

 reliable, on account of the changeability of its sensation zero. 

 We can localize temperature sensations much as tactile, but not 

 so accurately. 



The receptors for cold are near the surface of the skin; those 

 for warmth are imbedded deeply within it. While the latter 

 respond only to temperatures above their own, the cold receptors 

 are stimulated not only by temperatures below their own but 

 also by temperatures above 45 C. (140 F.). It is for this reason 

 that a sensation of cold is felt when one first steps into a hot bath; 

 the receptors for cold being nearer the surface than those for 

 warmth are stimulated an instant before them. It is said that the 

 sensation of "hot" as distinguished from "warm" results from 



