290 



Major W. B. Cannon. 



required to moisten the buccal and pharyngeal mucosa sufficiently to 

 extinguish thirst — a point made by Voit* — and that these regions become 

 dried rapidly when there is absence of an adequate water-content in the 

 body. This interpretation is consistent with the view that thirst is a sensa- 

 tion having a local source. Furthermore, this interpretation is not contra- 

 dicted by the satisfaction manifested by the dog after the fistula was closed, 

 for the water which is absorbed, like that injected into veins, may quench 

 thirst by altering local conditions. We cannot admit, therefore, that Bernard's 

 experiment is proof that thirst is a general sensation. 



Another set of observations cited as favourable to the theory of the 

 diffused character of the origin of thirst are those of Longet. After severing 

 the glosso-pharyngeal, the lingual and the vagus nerves on both sides in dogs, 

 he observed that they drank as usual after eating.f If thirst has a local 

 origin in the mouth and pliarynx, why should the animals in which the 

 nerves to these regions were cut still take water ? Two answers to this 

 question may be given. First, as Voit has pointed out,t Longet did not cut 

 all branches of the vagi and trigemini to the mouth and pharynx, and, conse- 

 quently, some sensation persisted. And second, even if all nerves were cut, 

 the fact that the animals drank would not prove that thirst exists as a 

 general feeling, for one may drink from the sight of tluid, or from custom, 

 without the stimulation of a dry mouth, just as one may eat from the sight 

 of food without the stimulus of hunger. In other words, the element of 

 appetite, previously considered, may enter, and as a matter of habit and 

 associated experience determine present reactions. 



The remaining evidence in favour of the diffused origin of thirst is found 

 in studies of blood changes. These changes, by altering the " milieu 

 int^rieur " of the body cells, must affect them all. In 1900, Mayer published 

 reports on the increase of osmotic pressure of the blood, as determined by 

 depression of the freezing point of the serum, which he noted in conditions 

 naturally accompanied by thirst. Dogs deprived of water for several days 

 had a blood serum in which the osmotic pressure was increased, and rabbits 

 kept in a specially warmed chamber showed the same change. Thus, condi- 

 tions in which the water supply to the body was stopped, or the loss of water 

 from the body by swe^iting or pulmonary evaporation was increased, either 

 of which is known to cause thirst, were associated with a rise of osmotic 

 pressure. And Mayer argued that all other circumstances in which thirst 

 appears — in diabetes with increased blood sugar, in renal disease with 



♦ Voit, ' Hermann's Handbuch der Physiologie,' Leipzig, Abth. 6, p. 566 (1881). 

 t Longet, ' Traite de Physiologie,' Paris, vol. 1, p. 3-5 et seq. (1868). 

 X Voit, loc. cit. 



