SENSATION OF TEMPERATURE. 223 



sensation is produced as when we touch one at a high tem- 

 perature. Contact with a ball of frozen mercury causes 

 a burning sensation, the same as that of iron heated to 

 100 C. (212 R), though we know that mercury freezes at 

 40 C. ( 40 F. ) Voyagers in the Polar regions are com- 

 pelled to envelop the metallic portions of their instruments in 

 non-conducting substances, so as to be able to handle them 

 with impunity. 



The skin and the mucous membranes do not appreciate 

 differences of temperature with equal nicety in every portion 

 of their surface, and the regions which are most sensitive to 

 contact are not those which best indicate the temperature. 

 The palmar surface of the fingers, the tongue, and the lips, 

 are less impressionable in this respect than the skin of the 

 cheeks, eyelids, elbow, and the pituitary membrane. We 

 may perhaps attribute this relative insensibility in the hand, 

 and the mucous membrane of the cheeks, to the habit of 

 contact with warm bodies. The hand becomes rapidly 

 inured to hold objects hot enough to cause a painful sensa- 

 tion to persons unaccustomed to it. We see this in chemists, 

 blacksmiths, and others. The skin need not necessarily be 

 thickened, although this condition also decreases the sensi- 

 bility. We often see persons of mature age bear without 

 pain the contact of food so hot that younger people could 

 not swallow it. The mucous membranes of the oesophagus 

 and the stomach are more sensitive in this respect than 

 that of the mouth ; but when the food is held immovable for 

 a few seconds between the palate and the tongue, the heat 

 is absorbed, and the food or drink may be swallowed with 

 impunity. 



It is clear from the preceding remarks that the sense of 

 touch is an unreliable thermometer; it is sufficient, however, 

 to guide us in matters relating to health, and in regard to 

 external objects, especially when we permit its full develop- 

 ment by touch. 



The hand is the principal means of exercising the touch. 

 The organization of this admirable instrument, its numerous 

 articulations, the freedom and variety of its movements, the 

 tactile sensibility which is so fully developed in the palmar 



