624 DIGESTION. 



long speaking or singing; certain kinds of diet as the saline and spicy, 

 and especially the habit, acquired by some, of drinking frequently. 

 Whilst individuals, thus circumstanced, may need several gallons a day 

 to satisfy their wants; others, who have, by resistance, acquired the 

 habit of using very little liquid, may be enjoying health and not expe- 

 riencing the slightest inconvenience from the privation of liquid; so com- 

 pletely are we, as regards the character and quantity of our aliment, 

 the creatures of habit. This privation, it is obvious, cannot be abso- 

 lute or pushed beyond a certain extent. There must always be fluid 

 enough taken to administer to the necessities of the system. 



As in the production of all sensations, three acts are required for 

 accomplishing that of thirst ; impression, conduction, and perception. 

 The last, as in every similar case, is effected by the brain; and the 

 second by the nerves passing between the part impressed and that 

 organ. The act of impression its seat and cause will alone arrest our 

 attention, and it will be found that we are still less instructed on these 

 points than on the physiology of hunger. Even with regard to the 

 seat of the impression, we are in a state of uncertainty. It appears to 

 be chiefly in the back part of the mouth and fauces ; but whether pri- 

 marily there, or experienced there by sympathy with the condition of 

 the stomach, is by no means clear. The latter opinion, however, 

 appears the more probable. In a remarkable case, published by Dr. 

 Gairdner of Edinburgh, it was found impracticable to allay thirst by 

 merely supplying the mouth, tongue, and fauces with fluid. A man 

 had cut through the oesophagus. An insatiable thirst arose ; several 

 pailfuls of water were swallowed daily, and discharged through the 

 wound without removing the desire for drink; but on injecting water, 

 mixed with a little spirit, into the stomach, if was soon quenched. 

 That the sensation is greatly dependent upon the quantity of fluid cir- 

 culating in the vessels is shown by the fact, mentioned by M. Dupuytren, 

 that he succeeded in allaying the thirst of animals, by injecting milk, 

 whey, water or other fluids into the veins; and M. Orfila states, that, in 

 his toxicological experiments, he frequently allayed in this way the 

 excessive thirst of animals, to which he had administered poison; and 

 which were incapable of drinking, owing to the oesophagus having been 

 tied. He found, also, in his experiments, that the blood of animals 

 was more and more deprived of its watery portions as the abstinence 

 from liquids was more prolonged. 1 



Like all other sensations, that of thirst arises from an inappreciable 

 modification of the nerves of the organ : hence, all the hypotheses 

 proposed to account for it have been mere fantasies undeserving of 

 enumeration. 



The prehension of liquids differs somewhat from that of solids. The 

 fluid may be simply poured into the mouth, or a vacuum may be formed 

 in it: the pressure of the atmosphere then forces it in. When we 

 drink from a vessel, the mouth is applied to the surface of the fluid; 

 the chest is then dilated, so as to diminish the pressure of the atmo- 

 sphere on the portion of the surface of the liquid intercepted by the 



1 Adelon, Physiologie de t'Homme, 2de 6dit., ii. 525, Paris, 1829. 



