The sensation of hunger is commonly referred to the stomach, and that of thirst to the upper part of the throat and back of the mouth; and correctly enough to this extent, that a certain condition of the stomach and throat tends to produce them. But, in reality, the sensations themselves, like all other mental affections and emotions, have their seat in the brain, to which a sense of the condition of the stomach is conveyed through the medium of the nerves. In this respect, Appetite resembles the senses of Seeing, Hearing, and Feeling; and no greater difficulty attends the explanation of the one than of the others. Thus, the cause which excites the sensation of colour, is certain rays of light striking upon the nerve of the eye; and the cause which excites the perception of sound, is the atmospherical vibrations striking upon the nerve of the ear; but the sensations themselves take place in the brain, to which, as the organ of the mind, the respective impressions are conveyed. In like manner, the cause which excites appetite is an impression made on the nerves of the stomach; but the feeling itself is experienced in the brain, to which that impression is conveyed. Accordingly, just as in health no sound is ever heard except when the external vibrating atmosphere has actually impressed the ear, and no colour is perceived unless an object be presented to the eye,--so is appetite never felt, except where, from want of food, the stomach is in that state which forms the proper stimulus to its nerves, and where the communication between it and the brain is left free and unobstructed. But as, in certain morbid states of the brain and nerves, voices and sounds are heard, or colours and objects are seen, when no external cause is present to act upon the ear or the eye,--so, in disease, a craving is often felt when no real want of food exists, and where, consequently, indulgence in eating can be productive of nothing but mischief. Such an aberration is common in nervous and mental diseases, and not unfrequently adds greatly to their severity and obstinacy. In indolent unemployed persons, who spend their days in meditating on their own feelings, this craving is very common, and from being regarded and indulged as if it were healthy appetite, is productive of many dyspeptic affections.[3]