The Physiological Basis of Thirst. 



287 



may be alleviated, as everyday practice demonstrates, by a moderate 

 quantity of water, or through exciting a flow of saliva by taking into the 

 mouth fruit acids sucli as lemon or tomato juice, or by chewing insoluble 

 substances. In the second stage the saliva and mucus in the mouth and 

 throat become scant and sticky, There is a feeling of dry deadness of the 

 mucous membranes. The inbreathed air feels hot. The tongue clings to the 

 teeth or cleaves to the roof of the mouth. A lump seems to rise in the 

 throat, and starts endless swallowing motions to dislodge it. Water and 

 wetness are then exalted as the end of all excellence. Even in this stage 

 the distress can be alleviated by repeatedly sipping and sniffing a few drops 

 of water at a time. " Many prospectors," McGee states, " become artists in 

 mouth moistening, and carry canteens only for this purpose, depending on 

 draughts in camp to supply the general needs of the system." The last 

 three stages described by McGee, in which the eyelids stiffen over eyeballs 

 set in a sightless stare, the distal tongue hardens to a dull weight, and the 

 wretched victim has illusions of lakes and running streams, are too patho- 

 logical for our present interest. 



The fact I wish to emphasise is the persistent dryness of the mouth and 

 throat in thirst. Direct testimony is given by King, a medical officer in a 

 United States Cavalry troop, which for 3J days was lost without water in 

 the torrid " Llano Estacado " of Texas. * He records that, on the third day, 

 salivary and mucous secretions had long been absent, and that mouths and 

 throats were so parched that food, on being chewed, gathered about the 

 teeth and in the palate, and could not be swallowed. " Sugar would not 

 dissolve in the mouth."* 



Further evidence of the relation between local dryness of the mouth and 

 throat and the sensation of thirst is found in some of the conditions which 

 bring on the sensation. Breathing hot air free from moisture, prolonged 

 speaking or singing, the repeated chewing of desiccated food, the inhibitory 

 influence of fear and anxiety on salivary secretion, have all been observed to 

 result in dryness of the buccal and pharyngeal mucous membrane and in 

 attendant thirst. On the other hand, conditions ai'ising in regions remote 

 from the mouth and involving a reduction of the general fluid content of the 

 body, such as profuse sweating, the excessive diarrhoea of cholera, the 

 diuresis of diabetes, as well as such losses as occur in hremorrhage and 

 lactation, are well recognised causes of the same sensation. There appear to 

 be, therefore, both local and general origins of thirst. In correspondence 

 with these observations, two groups of theories have arisen, just as in the 



* King, ' Amer. Joui'. Med. Sci.,' vol. 75, p. 404 (1878). 



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