SENSE OF TEMPERATURE 685 



too, our estimate of the size of a cavity in a tooth is usually exaggerated when 

 based upon sensations derived from the tongue alone. Another curious 

 illusion may here be mentioned. If we close the eyes, and place a small 

 marble or pea between the crossed fore and middle fingers, we seem to be 

 touching two marbles, figure 480. This illusion is due to an error of judg- 

 ment. The marble is touched by two surfaces which, under ordinary cir- 

 cumstances, could be touched only by two separate marbles, hence the mind, 

 taking no cognizance of the fact that the fingers are crossed, forms the con- 

 clusion that two sensations are due to two marbles. 



Sense of Temperature. The whole surface of the body is more or 

 less sensitive to differences of temperature. The sensation of heat is distinct 

 from that of touch, hence it would seem reasonable to suppose that there are 

 special nerves and nerve endings for temperature. At any rate the power of 

 discriminating temperature may remain unimpaired when the sense of touch 

 is temporarily in abeyance. Thus if the ulnar nerve be compressed at the 

 elbow till the sense of touch is very much dulled in the fingers which it sup- 

 plies, the sense of temperature remains quite unaffected. And in certain 

 diseases of the cord the sense of touch may be impaired in a part, and tem- 

 perature remain undisturbed, or the converse. 



FIG. 421. Diagram of a Part of the Hand, Showing Distribution of Sense Spots; for 

 touch, A ; for heat, B; and for cold, C. In A the skin is sensitive except at the parts marked 

 with black; in B and C, the intensity of the shading represents the relative sensitiveness. 

 (Goldscheider.) 



The mapping oi the surface of a part of the skin with reference to its 

 sensibility to temperature reveals the fact that there are definite heat and 

 cold spots. Furthermore, the areas do not coincide, leading us to conclude 

 that there are two distinct sense organs concerned, figure 421, B and C. 



The sensations of heat and cold are often exceedingly fallacious, and in 

 many cases are no guide at all to the absolute temperature as indicated by 

 a thermometer. All that we can with safety infer from our sensations of 

 temperature is that a given object is warmer or cooler than the skin. Thus 



