HUNGER and THIRST. These terms are used to express peculiar sensations which are produced by and give expression to general wants of the system, satisfied respectively by the ingestion of organic solids containing substances capable of acting as food, and by water or liquids and solids containing water.
Hunger (a word common to Teutonic languages) is a peculiarly indefinite sensation of craving or want which is referred to the stomach, but with which is often combined, always indeed in its most pronounced stages, a general feeling of weakness or faintness. The earliest stages are unattended with suffering, and are characterized as " appetite for food." Hunger is normally appeased by the introduction of solid or semi-solid nutriment into the stomach, and it is probable that the almost immediate alleviation of the sensation in these circumstances is in part due to a local influence, perhaps connected with a free secretion of gastric juice. Essentially, however, the sensation of hunger is a mere local expression of a general want, and this local expression ceases when the want is satisfied, even though no food be introduced into the stomach, the needs of the economy being satisfied by the introduction of food through other channels, as, for example, when food which admits of being readily absorbed is injected into the large intestine.
Thirst (a word of Teutonic origin, Ger. Durst, Swed. and Dan. torsi, akin to the Lat. torrere, to parch) is a peculiar sensation of dryness and heat localized in the tongue and throat. Although thirst may be artificially produced by drying, as by the passage of a current of air over the mucous membrane of the above parts, normally it depends upon an impoverishment of the system in water. And, when this impoverishment ceases, in whichever way this be effected, the sensation likewise ceases. The injection of water into the blood, the stomach, or the large intestine appeases thirst, though no fluid is brought in contact with the part to which the sensation is referred.
The sensations of hunger and thirst lead us, or when urgent compel us, to take food and drink Into the mouth. Once in the mouth, the entrance to the alimentary canal, the food begins to undergo a series of processes, the object of which is to extract from it as much as possible of its nutritive constituents. Food in the alimentary canal is, strictly speaking, outside the confines of the body; as much so as the fly grasped in the leaves of the insectivorous Dionea is outside of the plant itself. The mechanical and chemical processes to which the food is subjected have their seat and conditions outside the body which it is destined to nourish, though unquestionably the body is no passive agent, and innumerable glands come into action to supply the chemical agents which dissolve and render assimilable those constituents of the food capable of being absorbed into the organism, and of forming part and parcel of its substance (see further under NUTRITION).
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)