CHAP, vi.] SOME OTHER SENSATIONS. 285 



but a wound, we suffer pain, but do not recognize the sharpness or 

 the heat which is causing the pain. This suggests that the 

 special sensations of touch and temperature are brought about 

 by special, epithelial structures serving as the differentiated ends 

 of nerve fibres, but that common sensibility and pain need no 

 such special endings ; this however opens up questions which we 

 must consider separately by themselves. 



885. Hunger and thirst. We may introduce here the few 

 words that we have to say concerning two affections of consciousness, 

 which may perhaps be considered as kinds of sensation, 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 instance by being ex- 

 posed to too great an evaporation, we feel ' thirsty,' and the 

 feeling is at once removed by adequately moistening the membrane. 

 Under ordinary circumstances however 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 sweating 

 which in themselves do not cause special dryness of the mucous 

 membrane of the soft palate ; this part then undergoes a loss of 

 water in common with the other tissues, but not in a special 

 degree. Nevertheless thirst thus brought about may be tem- 

 porarily 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 deficiency 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 coming 

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

 fistula water is introduced into the stomach through the fistulous 

 opening, large quantities may be given before thirst is assuaged. 



The sensation of hunger is in a somewhat similar manner 

 referred to, or associated with, the condition of the gastric mucous 

 membrane. We feel hungry when the stomach is empty. But 

 even more distinctly than in the case of thirst the main cause 



