280 ON CUTANEOUS SENSATIONS. [BOOK m. 



ture are not identical with those most sensitive to variations 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 recog- 

 nize which region of the skin is being heated or cooled ; and thus 

 these sensations also enter into our perceptions of the external 

 world. 



881. We have treated of the sensations of touch and of heat 

 and cold as cutaneous sensations ; but they are not confined 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 oesophagus ; a fluid which in the mouth 

 is felt distinctly as hot gives rise to a sensation of pain not of 

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

 as cold or warm when swallowed in quantity sufficient to affect 

 by conduction the abdominal skin. The maintenance of these 

 cutaneous 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. 



