234 HUMAN PHYSIOLOGY. 



space or extension is obtained when the sensory surface or the ex- 

 ternal object changes its place in regard to the other; the character 

 of the surface, its roughness or smoothness, is estimated by the im- 

 pressions made upon the tactile papillae. 



Appreciation of Temperature. The general surface of the body is 

 more or less sensitive to differences of temperature, though this 

 sensation is separate from that of touch ; whether there are nerves 

 especially adapted for the conduction of this sensation has not been 

 fully determined. Under pathologic conditions, however, the sense 

 of touch may be abolished, while the appreciation of changes in tem- 

 perature may remain normal. 



The cutaneous surface varies in its sensibility to temperature in 

 different parts of the body, and depends, to some extent, upon the 

 thickness of the skin, exposure, habit, etc. ; the inner surface of the 

 elbow is more sensitive to changes in temperature than the outer 

 portion of the arm ; the left hand is more sensitive than the right, 

 the mucous membrane less so than the skin. 



Excessive heat or cold has the same effect upon the sensibility ; 

 the temperatures most readily appreciated are those between 50 F. 

 and 115 F. 



The sensations of pain and tickling appear to be conducted to the 

 brain, also, by nerves different from those of touch ; in abnormal 

 conditions the appreciation of pain may be entirely lost while touch 

 remains unimpaired. 



THE SENSE OF TASTE. 



The sense of taste is localized mainly in the mucous membrane 

 covering the superior surface of the tongue. 



The tongue is situated in the floor of the mouth ; its base is 

 directed backward and is connected with the hyoid bone and by 

 numerous muscles with the epiglottis and soft palate; its apex is 

 directed forward against the posterior surface of the teeth. 



The substance of the tongue is made up of intrinsic muscle-fibers, 

 the linguales ; it is attached to surrounding parts, and its various 

 movements are performed by the extrinsic muscles e. g., stylo- 

 glossus, geniohyoglossus, etc. 



The mucous membrane covering the tongue is continuous with that 



