The Physiological Basis of Thirst. 



295 



theory of thirst, on which I wish to offer evidence, may now be stated. 

 In brief, it is that the salivary glands have, among their functions, that of 

 keeping moist the ancient watercourse ; that they, like other tissues, suffer 

 when water is lacking in the body — a lack especially important for them, 

 however, because their secretion is almost wholly water, and that, when 

 these glands fail to provide sufficient fluid to moisten the mouth and 

 throat, the local discomfort and unpleasantness which result constitute the 

 feeling of thirst. 



That one of the uses of buccal glands is to keep wet the surfaces over 

 which their secretion is distributed is indicated by the fact that these 

 structures first appear in air- inhabiting vertebrates. This indication 

 receives support from the conditions seen in the cetacea, the mammalian 

 forms which have returned to an aquatic existence, and in which both the 

 water-loss from the bodv and the need for wettingr the mouth and throat are 

 greatly reduced. It is a remarkable fact that in these animals the salivary 

 glands are either lacking or are very rudimentary. The appearance and 

 disappearance of tlie buccal glands in large animal groups, in correspondence 

 with the exposure or non-expnsure of the mouth and throat to desiccating 

 air, point to these glands as protectors of the buccal mucosa against drying. 



Experimental evidence as to the protective function of the salivary 

 secretions was provided incidentally many years ago by Bidder and Schmidt. 

 They were interested in studying any fluid secretion which might appear in 

 the mouth apart from saliva. To this end they tied in dogs all the 

 salivary ducts. The first effect was such a striking diminution of the fluid 

 layer over the buccal mucosa that only when the mouth was held closed 

 was the surface kept moist, and, when the animal breathed through the 

 mouth, a real drying of the surface was hardly prevented. The eagerness 

 for water, they state, was enormously increased, so that the animal was 

 always ready to drink.* 



Related to this service of saliva in moistening and lubricating the mouth 

 parts is the presence of a special reflex for salivary secretion when the 

 buccal mucosa is exposed to conditions which tend to dry it. Thus, as 

 Pavlov'st researches demonstrated, with dry food in the mouth, much more 

 saliva is secreted than with moist food. And ZebrowskiJ found, in the 

 course of observations on patients with a parotid fistula, that, whereas no 

 saliva flowed with the mouth closed, as much as 0-25 c.c. in five minutes 

 •came from the duct when the mouth was opened. This reflex is readily 



* Bidder and Schmidt, ' Verdauungssafte und StoflFwechsel,' Leipzig, p. 3 (1852). 

 + Pavlov, ' The Work of the Digestive Glands,' London, 2nd ed., pp. 70, 82 (1910). 

 X Zebrowski, ' Arch. f. d. ges. Physiol.,' vol. 110, p. 105 (1905). 



