CHAP, vi.] SOME OTHER SENSATIONS. 1043 



We can with some accuracy distinguish small differences of 

 temperature, especially those lying near the normal tempera- 

 ture of the skin. In this respect these sensations follow 

 Weber's law, though apparently slight differences of cold ;tn- 

 more easily recognized than those of slight heat. The range 

 of the greatest sensitiveness seems to lie between 27 and 33. 



The regions of the skin most sensitive to variations in 

 temperature are not identical with those most sensitive to varia- 

 tions in pressure. Thus the cheeks, eyelids, temples and lips, 

 are more sensitive than the hands. The least sensitive parts 

 are the legs, and front and back of the trunk ; to this matter 

 however we shall return. 



As with pressure sensations so also with sensations of heat 

 and cold, two sensations excited at a certain distance apart may 

 or may not be fused into one, the distance necessary for the 

 separation of the sensations varying in different regions of the 

 body, and being, as might be expected from the ease with which 

 heat and cold are conducted, much greater than in the case of 

 pressure sensations. We also ' localize ' the sensations of heat and 

 cold ; we can recognize which region of the skin is being heated 

 or cooled ; and thus these sensations also enter into our percep- 

 tions of the external world. 



649. We have treated of the sensations of touch and of 

 heat and cold as cutaneous sensations ; but they are not con- 

 fined to the skin commonly so called. We experience the 

 same sensations in varying degree by help of the lining of the 

 mouth and pharynx, which is called a mucous membrane ; and 

 they may also be traced for a short distance up the rectum beyond 

 the margin of the skin proper. But in both these situations, 

 the lining membrane is by origin and in structure epiblastic, 

 that is to say cutaneous, and in possessing cutaneous functions 

 shews its real nature. These functions are most marked at the 

 beginning of the passages, the tip of the tongue being very 

 sensitive to touch and heat and cold, with a well-developed 

 power of localization ; they are very rapidly lost in the rectum, 

 and more gradually disappear at the lower part of the pharynx 

 and in the O3sophagus ; a fluid which in the mouth is felt dis- 

 tinctly as hot gives rise to a sensation of pain not of heat wln-n 

 it is swallowed, and a cold or warm drink is only felt as cold 

 or warm when swallowed in quantity sufficient to affect by con- 

 duction the abdominal skin. The maintenance of these cutane- 

 ous functions in the initial parts of the alimentary canal, which 

 are under the dominion of the will, is, like the sense of taste, a 

 safeguard against the introduction into the canal of noxious 

 substances ; in the subsequent parts, no longer subject to the 

 will, any warning which such sensations might give would be 

 too late and useless. 



