THE RECEPTOR SYSTEM 217 



if we could discriminate the excitations of each of these from that 

 of its immediate neighbors we would get the sensation of a series 

 of points touching us, one for each nerve region excited; but in 

 the absence of intervening unexcited nerve areas the sensations 

 are fused together. 



The Temperature Sense. By this we mean our faculty of 

 perceiving cold and warmth; and, with the help of these sensa- 

 tions, of perceiving temperature differences in external objects. 

 Its organ is the whole skin, the mucous membrane of mouth and 

 fauces, pharynx and upper part of gullet, and the entry of the 

 nares. Direct heating or cooling of a sensory nerve may stimulate 

 it and cause pain, but not a true temperature sensation; and the 

 amount of heat or cold requisite is much greater than that neces- 

 sary when a temperature-perceiving surface is acted upon; hence 

 we must assume the presence of temperature receptors. As 

 previously stated these are of two kinds, those that are stimulated 

 by cold, and those that are stimulated by warmth. 



In a comfortable room we feel at no part of the Body either 

 heat or cold, although different parts of its surface are at differ- 

 ent temperatures; the fingers and nose being cooler than the 

 trunk which is covered by clothes, and this, in turn, cooler than 

 the interior of the mouth. The temperature which a given region 

 of the temperature organ has (as measured by a thermometer) 

 when it feels neither hot nor cold is its temperature-sensation zero 

 for that time, and is not associated with any one objective tem- 

 perature; for not only, as we have just seen, does it vary in dif- 

 ferent parts of the organ, but also on the same part from time to 

 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 



