Many attempts have been made but without much success, to determine what the peculiar condition of the stomach is which excites in the mind the sensation of hunger. For a long time it was imagined that the presence of _gastric_ or _stomach_ juice, irritating the nerves of the mucous membrane, was the exciting cause; but it was at last ascertained, that, after the digestion of a meal is completed, and the chyme has passed into the intestine, the gastric juice ceases to be secreted till _after_ a fresh supply of food has been taken in.[5] It was next supposed that the mere emptiness of the stomach was sufficient to excite hunger, and that the sensation arose partly from the opposite sides rubbing against each other. But this theory is equally untenable; for the stomach generally contains a sufficient quantity of air to prevent the actual contact of its sides, and moreover it may be entirely void of food and yet no appetite be felt. It may be laid down, indeed, as a general rule, that an interval of rest must follow the termination of digestion before the stomach becomes fit to resume its functions, or appetite is experienced in any degree of intensity; and the length of time required for this purpose varies very much, according to the mode of life and to the extent of waste going on in the system. In many diseases, too, the stomach remains empty for days in succession, without any corresponding excitation of hunger. Even in healthy sedentary people, whose expenditure of bodily substance is small, real appetite is not felt till long after the stomach is empty, and hence one of their most common complaints is the want of appetite. Dr Beaumont suggests a distended state of the vessels which secrete the gastric juice as the exciting cause of hunger, and thinks that this view is strengthened by the rapidity with which the juice is poured out after a short fast--a rapidity, he says, which cannot be accounted for except by supposing the juice to have existed ready made in the vessels or follicles by which it is secreted. But this theory is not more satisfactory than the rest, for in the sudden flow of saliva into the mouth of a hungry man on the unexpected appearance of savoury viands, we have an instance of equally rapid secretion where there was evidently no storing up beforehand, Besides, there is an obvious relation between appetite and the wants of the system, which is not always taken sufficiently into account, and which is nevertheless too important to be overlooked.