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Major W. B. Cannon. 



action of the salivary glands, however, to the mechanism of the water supply 

 of the body, lies in the strategic position of these glands in relation to a 

 surface which tends to become dry by the passage of air over it. If this 

 surface is not kept moist, discomfort arises and with it an impulse to seek 

 well tried means of relief. Thus the diminishing activity of the salivary 

 glands becomes a delicate indicator of the bodily demand for fluid. 



The foregoing explanation is in agreement with the suggestions which have 

 been offered to account for thirst as having a local origin. But it does not 

 require specialised nerves, or peculiar sensitiveness of the first portion of the 

 digestive tract, which have been assumed to be present by the upholders of 

 this theory. And by calling attention to the arrangement by which the 

 salivary glands are made to serve as indicators of the general bodily need for 

 water, it piresents a reasonable account of the manner in which a widespread 

 condition of the organism may exhibit itself locally. 



The experiments which have long been the chief support of the theory 

 that thirst is a general sensation can also be explained by the evidence above 

 adduced. The abolition of thirst by injecting fluid into the veins of thirsty 

 animals would be expected, for, as shown in the experiment illustrated in 

 fig. 1, by providing an adequate water supply the saliva flow is promptly 

 re-established, and the parched mouth and throat are again continuously 

 moistened. In the classic experiment of Claude Bernard the animal with an 

 open gastric fistula continued to drink until the fistula was closed. This was 

 not because there was a general demand for water throughout the body, so 

 long as the fistula remained open, but because only when escape through the 

 fistula was stopped did the body receive the water needed to provide the 

 output of saliva which prevented local drying. And the dogs with salivary 

 glands tied, described by Bidder and Schmidt, were always ready to drink, 

 just as are persons who are terrified or who have been given atropine, 

 because of thirst — because there is local drying of the mouth — from lack of 

 saliva, though the body as a whole may not be in any need of water. The 

 application of cocaine to the mucous surfaces of the mouth abolishes the 

 torment of thirst, not by any central effect, and clearly not by satisfying any 

 general bodily requirement for water, but by rendering the surfaces 

 ansesthetic. The miraculous virtues of coca leaves, as a balm for the distress 

 of the thirsty, a fact long ago observed, is explicable on these grounds. The 

 thirst of those who suffer from loss of fluid from the body — the diabetic 

 patient, the victim of cholera, the subject of haemorrhage, the perspiring 

 labourer, and the nursing mother — can be accounted for by the reduction of 

 salivary flow as the water-content of the body is lowered, and by the conse- 

 quent discomfort arising from the sticky buccal mucosa. 



