1048 ON CUTANEOUS AND [Book in. 



dermis or to the nerves running in it, we still experience 

 sensations of pain, though no longer those of touch and tem- 

 perature; when a sharp or hot body is made to touch, not 

 the intact skin but a wound, we suffer pain, but do not rec- 

 ognize the sharpness or the heat which is causing the pain. 

 This suggests that the special sensations of touch and tem- 

 perature are brought about by special, epithelial structures 

 serving as the differentiated ends of nerve fibres, but that com- 

 mon sensibility and pain need no such special endings ; this 

 however opens up questions which we must consider separately 

 by themselves. 



§ 653. Hunger and thirst. We may introduce here the few 

 words that we have to say concerning two affections of con- 

 sciousness, which may perhaps be considered as kinds of sensa- 

 tion, namely, hunger and thirst. 



We refer our feelings of thirst to, or at least we associate 

 them with, a particular condition of the mucous membrane of 

 the mouth, especially of the soft palate. When the mucous 

 membrane of this region becomes drier than normal, as for in- 

 stance by being exposed to too great an evaporation, we feel 

 ' thirsty,' and the feeling is at once removed by adequately 

 moistening the membrane. Under ordinary circumstances how- 

 ever the condition of thirst is brought about, not by anything 

 bearing specially or exclusively on the mucous membrane of the 

 soft palate or even of the whole mouth, but by the diminution 

 of the water present in the body either through restriction of 

 the intake, or through excess of the output in the secretions, 

 such as that of sweat, or through both together. This is often 

 spoken of as diminution of the water of the blood ; but most 

 probably the specific gravity of the blood is kept constant by 

 the withdrawal of water from the lymph, so that the loss falls 

 on the latter fluid. Such a diminution of the water of the body 

 may be brought about by circumstances such as excessive sweat- 

 ing which in themselves do not cause special dryness of the 

 mucous membrane of the soft palate; this part then under- 

 goes a loss of water in common with the other tissues, but 

 not in a special degree. Nevertheless thirst thus brought about 

 may be temporarily assuaged by simple moistening of the soft 

 palate. From this we may infer that the sensation of thirst is 

 brought about by afferent sensory impulses started in the 

 mucous membrane of the soft palate by a deficiencjr of water in 

 that membrane, perhaps by a drain on the lymph spaces of that 

 membrane. 



We are in the habit of assuaging thirst by drinking water, 

 or watery fluids, and in doing so produce both a direct local 

 effect on the palate and a general indirect effect on the body. 

 In the absence of the local effect, the indirect effect is slow in com- 

 ing and needs a large quantity of fluid ; when in cases of gastric 



