The Physiological Basis of Thirst. 



289 



mediate the sensation of thirst is unquestioned. But the problem again is 

 presented, How are they stimulated ? 



The view that thirst is a general sensation was well - stated by Sehiff. It 

 arises, he declared, from a lessened water-content of the body, a condition 

 from which the whole body suffers. The local reference to the pharynx, 

 Hke the local reference of hunger to the stomach, is due to association of 

 experiences. Thus the feeling of dryness in the throat, though it accom- 

 panies thirst, has only the value of a secondary phenomenon, and bears no 

 deeper relation to the general sensation than heaviness of the eyelids bears 

 to the general sensation of sleepiness.* The conception of thirst, as a 

 general sensation, is commonly accepted, and is supported by considerable 

 experimental evidence. The interpretation of this evidence, however, is 

 open to question, and should be examiaed critically. 



First among the experiments cited are those of Dupuytren and the later 

 similar experiments of Orfila.f These observers abolished thirst in dogs by 

 injecting water and other liquids into the dogs' veins. And Schiff quotes 

 Magendie as having treated successfully by the same procedure the thirst of 

 a patient suffering from hydrophobia. In these instances the treatment was 

 no doubt general, in that it affected the body as a whole. But the assump- 

 tion that thirst is thus proved to be a general sensation is unwarranted, for 

 the injection of fluid into the circulation may have changed local conditions 

 in the mouth and pharynx, so that the local sensation no longer arose. 



A classic experiment repeatedly cited in the literature of thirst was one 

 performed by Claude Bernard. He opened a gastric fistula which he had 

 made in a dog, and allowed the water which the animal drank to pass out. 

 As the animal became thirsty, it would drink until " fatigued,"- as the report 

 states, and when " rested " it would begin again. But after the fistula was 

 closed, drinking quickly assuaged the desire for water. The inference was 

 drawn that thirst must be a general sensation, for the passage of water 

 through the month and pharynx wet those surfaces, and yet the animal was 

 not satisfied until the water was permitted to enter the intestine and be 

 absorbed by the body.j This evidence appears conclusive. The expressions 

 " fatigued " and " rested," however, are interpretations of the observer, and 

 not the testimony of the dog. Indeed, we may with equal reasonableness 

 assume that the animal stopped drinking because he was not thirsty, and 

 started again when he became thirsty. The only assumptions necessary for 

 such an interpretation of the animal's behaviour are that appreciable time is 



* Schiff, ' Physiologie de la Digestion,' Florence and Turin, vol. 1, p. 41 (1867). 

 t See ' Dictionnaire des Sci. Med.,' IParis, vol. 61, p. 469 (1821). 

 I Bernard, ' Physiologie Expeiimentale,' Paris, vol. 2, p. 49 (1856). 



