210 HUMAN PHYSIOLOGY. 



from that of touch; whether there are nerves especially adapted for the con- 

 duction of this sensation has not been fully determined. Under pathologic 

 conditions, however, the sense of touch may be abolished, while the apprecia- 

 tion of changes in temperature 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 temper- 

 atures 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 back- 

 ward 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 lin- 

 guales; it is attached to surrounding parts, and its various movements 

 are performed by the extrinsic muscles e. g., styloglossus, geniohyo- 

 glossus, etc. 



The mucous membrane covering the tongue is continuous with that lining 

 the commencement of the alimentary canal, and is furnished with vascular and 

 nervous papillae. 



The papilla are analogous in their structure to those of the skin, and 

 are distributed over the dorsum of the tongue, giving it its characteristic 

 roughness. 



There are three principal varieties 



1. The filiform papilla are most numerous, and cover the anterior two thirds 

 of the tongue; they are conic or filiform in shape, often prolonged into 

 filamentous tufts, of a whitish color, and covered by horny epithelium. 



2. Thefungiform papilla are found chiefly at the tip and sides of the tongue; 

 they are larger than the preceding, and may be recognized by their deep 

 red color. 



