The analogy between the external senses and the appetite is in various respects very close. If we are wrapt in study, or intent on any scheme, we become insensible to impressions made on the ear or eye. A clock may strike, or a person enter the room, without our being aware of either event. The same is the case with the desire for food. If the mind is deeply engaged, the wants of the system are unperceived and unattended to—as was well exemplified in the instance of Sir Isaac Newton, who, from seeing the bones of a chicken lying before him, fancied that he had already dined, whereas, in reality, he had eaten nothing for many hours. Herodotus ascribes so much efficacy to mental occupation in deadening the sense of hunger, that he speaks of the inhabitants of Lydia having successfully had recourse to gaming as a partial substitute for food, during a famine of many years continuance. In this account there is, of course, gross exaggeration; but it illustrates sufficiently well the principle under discussion.
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.
If the body be very actively exercised, and a good deal of waste be effected by perspiration and exhalation from the lungs, the appetite becomes keener, and more urgent for immediate gratification; and if it is indulged, we eat with a relish unknown on other occasions, and afterwards experience a sensation of bien-être or internal comfort pervading the frame, as if every individual part of the body were imbued with a feeling of contentment and satisfaction, the very opposite of the restless discomfort and depression which come upon us, and extend over the whole system, when appetite is disappointed. An amusing example of the principle here inculcated is to be found in the Correspondance Inedite de Madame du Deffand,[6] where she describes her friend Madame de Pequigni as an insatiable bustling little woman who consumes two hours every day in devouring her dinner, and “eats like a wolf.” But, then, remarks Madame du Deffand by way of explanation, “Il est vrai qu’elle fait un exercice enragé.”
There is, in short, an obvious and active sympathy between the condition and bearing of the stomach and those of every part of the animal frame—in virtue of which, hunger is felt very keenly when the general system stands in urgent need of repair, and very moderately when no waste has been suffered. This principle is strikingly illustrated during recovery from a severe illness. “In convalescence from an acute disease,” as is well remarked by Brachet, “the stomach digests vigorously, and yet the individual is always hungry. This happens because all the wasted organs and tissues demand the means of repair, and demand them from the stomach, which has the charge of sending them; and, therefore, they keep up in it the continual sensation of want, which, however, is, in this case, only sympathetic of the state of the body.”[7] In alluding to this subject, Blaine observes, that “Hunger and thirst can only be satisfactorily explained by considering them as properties in the stomach by which it sympathizes with the wants of the constitution; and hence it is, that food taken in invigorates, even before it can be digested.”[8] Hence also the prostration of strength that is felt when the stomach has been for some time empty.
This sympathy is sometimes singularly manifest even in disease. In some cases of affection of the mesenteric glands, for example, where stomachic digestion remains for a time pretty healthy, and the general system suffers chiefly from the want of nourishment caused by the passage of the chyle into the blood being obstructed, the appetite continues as keen and often keener than before; because the system, being in want of nourishment, and the stomach healthy, all its natural causes continue to act as before; and accordingly, when food is taken, it is digested there as usual, but the chyle which is formed from it in the intestine can no longer be transmitted through the swollen glands in its usual healthy manner, to be converted into nutritive blood in the lungs; and the system thus failing to receive the required supply, recommences its cravings almost as soon as if no food had been obtained. When the disease has advanced a certain length, however, fever springs up, and destroys both appetite and digestion.
The effects of exercise, also, shew very clearly the connexion between appetite and the state of the system. If we merely saunter out for a given time every day, without being actively enough engaged to quicken the circulation and induce increased exhalation from the skin and lungs, we come in with scarcely any change of feeling or condition; whereas, if we exert ourselves sufficiently to give a general impetus to the circulation, and bring out moderate perspiration, but without inducing fatigue, we feel a lightness and energy of a very pleasurable description, and generally accompanied by a strong desire for food. Hence the keen relish with which the fox-hunter sits down to table after a successful chase.
This intimate communion between the state of the system and that of the stomach is a beautiful provision of Nature, and is one of the causes of the ready sympathy which has often been remarked as existing between the stomach and all the other organs—in other words, of the readiness with which they accompany it in its departure from health, and the corresponding aptitude of their disorders to produce derangement of the digestive function. Apparently for the purpose, among others, of thus intimately connecting the stomach with the rest of the system, it is supplied with a profusion of nervous filaments of every kind, which form a closely-interwoven nervous network in its immediate neighbourhood, and the abundance of which accounts for the severe and often suddenly fatal result of a heavy blow on the pit of the stomach.
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.