The diminution of the fluid part of the body, is the cause of an uneasy sensation, indicating the necessity of repairing the waste of fluids, which we familiarly call

thirst. This is a sensation connected with some natural state of the corporeal funo tions, and altogether independent of the occasional excitement of foreign bodies, although it may be induced by these. There is a demand for a certain supply of liquid which is the result of repletion of the stomach, and the cause of our drinking

at our ordinary meals, but this is different from true or spontaneous thirst. True thirst occurs, when we have been some time without taking drink, (unless the food has consisted mainly of fruits and other succulent vegetables ; under which circumstances, a person may go for months without any desire for drink); when the system

has been greatly excited, whether by corporeal or mental causes ; when acid substances, particularly saline bodies, have been taken into the stomach ; and, in short,

in every condition of the system, which favors the inordinate excretion of fluids.

The immediate cause of thirst appears to be a dry state of the mouth and fauces ;

owing to the mucus which covers these parts becoming thick and viscid, though physiologists are not agreed on this point. This may arise from the absorption of the fluid parts of the saliva ; for it appears to be necessary for the due performance of the functions of the palate and the tongue, that the mucus should possess a certain degree of liquidity. The sensation of thirst is generally indicative of the necessity of a supply of fluid to the system generally ; for although thirst may be momentarily assuaged by wetting the mouth, or holding a thin fluid in it --yet it