292 



Major W, B. Cannon. 



slightly hypertonic, a dog may drink normal salt solution without lowering 

 his osmotic pressure and afterwards, by refusing further drink, act quite as 

 if he had slaked his thirst. But if an animal with a very hypertonic blood 

 is placed before hypertonic salt solution he takes it again and again — an 

 action which may be explained by a draining of water from the tissues with 

 increasing intensity, and a consequent increasing thirst. 



From all these olxservations Wettendorff concluded that the origin of thirst 

 does not reside in alterations of the blood itself, but in the act of withdraw- 

 ing water from the tissues. The liquids bathing the cells, therefore, would 

 be first to concentrate as water is lost from the organism. And since the 

 conditions of cellular life would thus be modified in all the tissues, the 

 peculiar state would develop which occasions the sensation of thirst. This 

 effect is generally diffused, and is independent of any peculiar influence of 

 the process of dehydration on the nervous system itself. 



In accounting for the localising of the sensation in the mouth and throat 

 Wettendorff distinguished between a " true thirst and a " false thirst." 

 " True thirst," he declared, is dependent on an actual bodily need, and is 

 persistent until the need is satisfied. " False thirst " is only a dryness of 

 the mouth and pharynx. Dryness in this region occurs, to be sure, in true 

 thirst, but it is then an expression of the general dehydration of the tissues, 

 exaggerated perhaps by contact with the outer air. Through experience the 

 two conditions — buccal dryness and general deliydration — have become 

 associated. Even in true thirst we may temporarily abolish the sensation by 

 moistening the pharyngeal mucous membrane, but the result is only a " false 

 satisfaction," a self-deception, made possible because long and pleasant 

 experience has proved that moistening this region by drink leads to the 

 satisfaction of an instinctive need.* 



The foregoing review of observations and theories has revealed that 

 the attitude of physiologists with reference to thirst has been much 

 as it was with reference to hunger. In each condition a general bodily 

 need has arisen from a lack of essential bodily material and is 

 signalled by a well-defined sensation. In each the testimony of ingenuous 

 persons regarding their feelings has been carefully set down, and then 

 explained away. Thus in the case of thirst the primary sensation is described 

 universally as an experience of dryness and stickiness in the mouth and 

 throat.f Instead of attempting to account for the experience as such, 



* Wettendorf, 'Travaux du Laboratoire de I'lnstitut Solvay,' Brussels, yoI. 4, 

 pp." 353-484 (1901). 



t Foster, ' Textbook of Physiology,' London, p. 1423 (1891); Ludvvig, ' Lehrbuch der 

 Physiologie,' vol. 2, p. .586 ; Voit, ' Hermann's Handbuch der Physiologic,' Abth. 6, p. 566. 



