Thirst, the sensation by which the want of fluid in the system is made known, dependent on the condition of the stomach, throat, and fauces, and in a state of health a tolerably faith-f ul indication of the requirements of the body. It is generally considered as immediately resulting from an impression on the nerves of the stomach, as it is allayed by the introduction of liquids through a tube, so that the fauces are not touched; in this way speedy relief is obtained, because of the instantaneous absorption of the fluid by the veins of the stomach. This, however, must be taken with some qualification, as the intensity of thirst bears no necessary relation to the amount of liquid in the stomach, but indicates a want of the system which can be supplied through the blood vessels, the rectum, or the skin; in fact, the conditions are very analogous to those mentioned under Hunger. A supply of fluid, as indicated by thirst, is necessary to make up for the losses by cutaneous and pulmonary exhalation, and by the urinary and other secretions, which are effected chiefly at the expense of the water in the blood.

Thirst is greatest in a dry and hot air, when the perspiration and other secretions are excessive; salted or highly spiced food, strong fermented liquors, and irritating substances and poisons applied to the intestinal mucous membrane, excite thirst, no doubt to induce an ingestion of fluid by which they may be diluted. A sudden loss of blood, either by the lancet or from a wound, or a rapid drain on the vascular system, as in Asiatic cholera, diarrhoea, or diabetes, causes thirst in proportion to its amount. Thirst is less when the food is watery, and when liquid can bo absorbed by the skin from the surrounding air or water. The thirst of fever does not necessarily indicate a pressing demand for fluids, but depends on the dryness and heat of the throat, mouth, and skin, with diminished transpiration, and is better relieved by small pieces of ice than by copious draughts of water. Animals with naked skins, like batrachians, living in water or moist air, have no need to drink to quench thirst, cutaneous absorption supplying the necessary fluid. The camel, on the other hand, supports thirst for a long time, carrying a supply of fluid in the water stomach.

The daily quantity of fluid taken by a man depends upon the temperature, the amount of exercise and perspiration, and the nature of the food. As a general rule, a man in good health, taking a fair amount of bodily exercise, consumes about 2½ lbs. of solid food and rather less than 3 lbs. of liquid food in a day. But as most articles of solid food, such as meat, bread, and vegetables, themselves contain a notable proportion of water, they contribute so far to satisfy the daily demand for liquids. The body requires to be supplied, either in the solid or liquid form, with about 4½ lbs. of water daily; and if this quantity from any cause be deficient, the sensation of thirst indicates the requirements of the system. Every loss of water from the body, as above mentioned, increases this demand. Consequently, when from any accidental circumstance, as in cases of shipwreck, a sufficient supply of water for drink cannot be obtained, every care should be taken to prevent the loss of fluids from the system.

The avoidance of perspiration, the protection of the skin from evaporation, and frequent bathing in salt water when practicable, are all efficient in diminishing the danger, and in protracting as long as possible the period of endurance.