Without pretending to determine what the precise condition of the nerves of the stomach is, which, when conveyed to the brain, excites the sensation of appetite, I think it sufficient for every practical purpose if we keep in mind, that _the co-operation of the nervous system is necessary for the production of appetite_, and that _there is a direct sympathy between the stomach and the rest of the body_, by means of which the stimulus of hunger becomes unusually urgent where the bodily waste has been great, although a comparatively short time has elapsed since the preceding meal. Appetite, then, being given for the express purpose of warning as when a supply of food is necessary, it follows that its call will be experienced in the highest intensity when waste and growth--or, in other words, the operations which demand supplies of fresh materials--are most active; and in the lowest intensity when, from indolence and the cessation of growth, the demand is least. In youth, accordingly, when bodily activity is very great, and a liberal supply of nourishment is required both to repair waste and to carry on growth, the appetite is keener and less discriminating than at any other period of life, and, what is worthy of remark, as another admirable instance of adaptation, _digestion is proportionally vigorous and rapid_, and abstinence is borne with great difficulty; whereas, in mature age, when growth is finished and the mode of life more sedentary, the same abundance of aliment is no longer needed, the appetite becomes less keen and more select in its choice, and digestion loses something of the resistless power which generally distinguishes it in early youth. Articles of food which were once digested with ease are now burdensome to the stomach, and, if not altogether rejected, are disposed of with a degree of labour and difficulty that was formerly unknown. Abstinence also is now more easily supported. When, however, the mode of life in mature age is active and laborious, and the waste matter thrown out of the system is consequently considerable, the appetite for food and the power of digesting it are correspondingly strong; for in general it is only when the mode of life is indolent and inactive, and the waste consequently small, that the appetite and digestion are weak. So natural, indeed, is the connexion between the two conditions, that _exercise_ is proverbially the first thing we think of recommending to improve the appetite and the tone of the digestive organs, when these are observed to be impaired; and where positive disease does not exist, no other remedy is half so effectual. But, as already noticed, exercise to be beneficial must be of a description calculated to increase the activity of the secretions and excretions; otherwise it cannot place the system in a condition to require an abundant supply or excite vigorous digestion.