134 HUMAN PHYSIOLOGY. 



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 

 pathological conditions, however, the sense of touch may be abolished, 

 while the appreciation of changes in temperature may remain normal. 



This 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 sensi- 

 tive 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 1 15 F. 



The sensation 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 connected with the hyoid bone, by numerous muscles, with 

 the epiglottis and soft palate ; its apex is directed forward against the pos- 

 terior surface of the teeth. 



The substance of the tongue is made up of intrinsic muscular fibres, 

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

 ments performed by the extrinsic muscles, e. g., stylo-glossus, genio-hyo- 

 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. Tht filiform papilla are most numerous, and cover the anterior two- 

 thirds of the tongue ; they are conical or filiform in shape, often prolonged 

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



2. The fungiform papilla are found chiefly at the tip and sides of the 



