The Physiological Basis of Thirst. 



291 



accumulation of waste material in the body fluids, in acute rabies with total 

 deprivation of water, in cholera with excessive outpouring of water into the 

 intestine — the osmotic pressure of the blood would he augmented. Moreover, 

 when a thirsty dog drinks, the hypertonicity of his serum disappears, his 

 normal condition is restored, and he stops drinking. 



By these observations Mayer was led to the conclusion that whenever the 

 osmotic pressure of the blood rises above normal, thirst appears ; when- 

 ever it returns to normal, thirst vanishes ; and as the pressure varies, thirst 

 also varies. Since intravenous injections of hypertonic salt solution cause, 

 by stimulation of the bulbar centres, according to Mayer, a rise of arterial 

 pressure and renal and intestinal vasodilation — both operating to lower the 

 abnormally high osmotic pressure of the blood — he infers that other agencies 

 are present in the organism besides the desire for water, which tend to keep 

 the blood normal. Thirst, he declares, is the last of a series of mechanisms 

 acting to protect the organism against hypertonicity of its fluids. 



In summary, then, the thirsty individual has a blood with high osmotic 

 pressure. This condition affects all the cells of the body. It disturbs the' 

 cells of the central nervous system, and thus leads both to protective 

 circulatory reactions and, in case these fail, to malaise and irritability, and a 

 reference of unpleasantness to the region of the pharynx. Accompanying 

 this, there is the impulse to drink, and when tliat is satisfied, the water taken 

 in restores the normal state.* 



Mayer's observations were soon confirmed, but his inferences were 

 challenged. In 1901, Wettendorfi", working in Brussels, reported that if dogs 

 are deprived of water their blood does, indeed, develop a hypertonicity, as 

 Mayer had found, but that this is a phenomenon which does not occur to 

 any marked degree in the first days of the deprivation. In one instance 

 there was no change in the. freezing point of the serum during three days of 

 thirst. Serious alteration of the osmotic pressure of the blood, therefore, is 

 comparatively tardy in its appearance. Since the organism is continually 

 losing water, and, nevertheless, the blood remains for a day or two unchanged, 

 Wettendorflf concluded that the consistency of the blood is preserved as long 

 as possible by withdrawal of water from the extravascular fluids and the 

 tissues. Further, thirst is clearly demonstrable long before any considerable 

 change in the blood is evident. One animal in which the freezing point of 

 the serum had been lowered only 0*01° C. by four days' deprivation of water, 

 drank 200 c.c. of physiological salt solution, a liquid which to the dog in 

 normal condition is quite repugnant. Again, when the blood has become 



♦ Mayer, ' C. E. Soc. de Biol.,' vol. 52, pp. 154, 389, 522 (1900) ; also ' Easai sur la Soif,' 

 from the Laboratory of Experimental Pathology, Faculty of Medicine, Paris, 1900. 



