The very possession of a stomach or receptacle, into which food sufficient for a shorter or longer period can be introduced at one time, and which we have already remarked as characterizing all animals from the lowest to the highest, almost necessarily implies the co-existence of some watchful monitor, such as appetite, to enforce attention to the wants of the system, with an earnestness which it shall not be easy to resist. If this were not the case in man, for example--if he had no motive more imperative than reason to oblige him to take food--he would be constantly liable, from indolence and thoughtlessness, or the pressure of other occupations, to incur the penalty of starvation, without being previously aware of his danger. But the Creator, with that beneficence which distinguishes all His works, has not only provided an effectual safeguard in the sensations of hunger and thirst, but moreover, attached to their regulated indulgence a degree of pleasure which never fails to insure attention to their demands, and which, in highly civilized communities, is apt to lead to excessive gratification. Such being the important charge committed to the appetites of hunger and thirst, it will be proper to submit to the reader, before entering upon the consideration of the more complicated process of digestion, a few remarks on their nature and uses. CHAPTER II. THE APPETITES OF HUNGER AND THIRST. Hunger and Thirst, what they are--Generally referred to the stomach and throat, but perceived by the brain--Proofs and illustrations--Exciting causes of hunger--Common theories unsatisfactory--Hunger sympathetic of the state of the body as well as of the stomach--Uses of appetite--Relation between waste and appetite--Its practical importance--Consequences of overlooking it illustrated by analogy of the whole animal kingdom--Disease from acting in opposition to this relation--Effect of exercise on appetite explained--Diseased appetite--Thirst--Seat of Thirst--Circumstances in which it is most felt--Extraordinary effects of injection of water into the veins in cholera--Uses of thirst, and rules for gratifying it.