THE RECEPTOR SYSTEM 181 



It is probable that the nerve areas are much smaller than the 

 tactile; and that several unstimulated must intervene between 

 the excited, in order to produce sensations which shall be dis- 

 tinct. If we suppose twelve unexcited nerve areas must inter- 

 vene, then, in Fig. 67, a and 6 will be just on the limits of a single 

 tactile area; and no matter how the points are moved, so long as 

 eleven, or fewer, unexcited areas come between, we would get a 

 single tactile sensation; in this way we can explain the fact that 

 tactile areas have no fixed boundaries in the skin, although the 

 nerve distribution in any part must be constant. We also see 

 why the back of a knife laid on the surface causes a continuous 

 linear sensation, although it touches many distinct nerve areas; 

 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 



