113. When the stomach is in a healthy condition, and has remained for some time empty, the well-known sensation of hunger is produced; to account for which, various hypotheses have been devised. Some have attributed its origin to the friction of the sides of the stomach upon each other, or to the dragging of the liver upon the diaphragm; others to the action of bile or acid vapours upon the stomach; to the compression of the nerves, or to the fatigue of the contracted fibres of the stomach: but such theories are subverted by the fact, that the stomach may remain empty for a long interval, during disease, without any sensation of hunger; and that, when present, it may cease or be allayed by various causes, although food should not have been taken; as often happens after the accustomed period of repast is over, or from the sudden communication of news that overwhelms us with grief or disappointment. Sleep also allays it. The physiologists of the present day attribute the phenomenon to the stimulant action of the gastric juice upon the nerves of the stomach; and to support this opinion, Dr. Wilson Philip relates the following experiment.

A person in good health was prevailed upon to abstain from eating for more than twenty-four hours, and during that interval to increase the appetite by more than ordinary exercise. At the end of this time he was extremely hungry; but, instead of eating, he excited vomiting by drinking warm water, and irritating the fauces. The water returned mixed only with a ropy fluid, such as the gastric juice is described to be. After this operation, not only all desire to eat was removed, but a degree of disgust was excited by seeing others eat. He, however, was prevailed upon to take a little bread and milk, which in a very short time ran into the acetous fermentation, as indicated by flatulence and acid eructation. I do not mean to deny that the presence of a portion of gastric juice may not contribute to the sensation of hunger; but I feel more disposed to refer the phenomenon to a peculiar state of the gastric nerves, in consequence of a species of indirect sympathy with the waning powers of the body, by which their peculiar sensibilities are exalted in proportion as those of other parts of the system decline, and thus are they constituted monitors, to demand the supplies that are essential for the general resuscitation.

With regard to the experiment of Dr. Philip, as above described, we may observe that it merely goes to show that disturbance of the stomach will destroy appetite for food; the existence of nausea, short of vomiting, will produce the same effect, and yet in this latter case the gastric juice is not removed; nay, we know from actual experiment that nausea is wholly independent of the stomach, for the sensation, and the spasm of retching, may be produced by the injection of tartar emetic into the veins of an animal, from which the stomach has been previously removed. Nausea, then, although like hunger it be referred to the stomach, is evidently independent of it; and if nausea can exist, why may not hunger also, even though the animal should have lost its stomach by excision?

114. With respect to the actual quantity of gastric fluid in an empty stomach, some doubts have arisen. It seems most probable that it is supplied only during digestion1, and that its secretion corresponds with the nature and quantity of the ingesta. If a narcotic be applied to the nerves, their power is paralyzed, and the sensation of hunger ceases; such an effect is produced by the juice of tobacco, although by long habit the stomach may become indifferent to its operation. Whenever the Indians of Asia and America undertake a long journey, and are likely to be destitute of provisions, they mix the juice of tobacco with powdered shells, in the form of small balls, which they retain in their mouths, the gradual solution of which serves to counteract the uneasy craving of the stomach. In like manner we may explain the operation of spirit in taking away the appetite of those who are not accustomed to it; while those who indulge the habit receive its stimulant without its narcotic impression. Lord Byron entertained a great dread of becoming corpulent, and on that account frequently abstained from food for several days together, appeasing the cravings of hunger by a wafer and a glass of brandy1.

1 See the Experiments of Dr. Beaumont, at page 37.

115. In farther proof of the direct relation between the sense of appetite and the healthy influence of the brain, it may be stated, that in certain cases of idiotcy the individuals are so low in the scale of intelligence, as not to manifest any desire for food, although they will eat and digest, when fed. From the conclusive experiments of M. Brochet of Lyons, we receive additional evidence of our theory of hunger. This intelligent physiologist kept a dog without any food for twenty-four hours, when it had become extremely ravenous; he then divided the par-vagian, and placed meat before it, but the animal, which had just before betrayed signs of the greatest impatience for a meal, lay quietly down. When the meat was brought into contact with its mouth, it began to eat, and continued to do so, without any reference to the quantity received by the stomach: in fact, the tie between that organ and the brain had no longer any existence, and the animal, therefore, influenced only by the gratification of the sense of taste, continued to eat until the gullet could no longer receive a supply. 116. Natural appetite, which is only the first degree of hunger, never appears to recur until the aliment previously introduced has been duly assimilated.

It cannot, therefore, strictly speaking, be said to have an immediate reference to the state of the stomach; for although all the chyme may have long since passed out of that organ, if any delay occurs in its ulterior changes, appetite will not return, for the nervous energy is engaged in their completion, and cannot therefore accumulate in the stomach: on the contrary, in certain diseases, as in tabes mesenterica, notwithstanding the presence of alimentary matter in the stomach, the appetite is never pacified, in consequence, probably, of the gastric nerves not receiving that satisfaction which is the necessary consequence of healthy digestion. The circumstance of young persons daily losing flesh, while the appetite for food remained vigorous, appeared to the ancient physicians so extraordinary, that Sennertus, Etmuller, and others, have gravely inquired whether marasmus might not be owing to magical incantation. Voracity, or canine appetite, may sometimes depend upon a morbid state of the pylorus, which suffers the food to pass out of the stomach before it is properly chymified: such cases are attended with extreme emaciation. It may also arise from nervous irritation, produced by accumulations in the colon.

From these views we may deduce the following important corollary - that the several processes by which aliment is converted into blood cannot be simultaneously performed, without such an increased expenditure of vital energy as weak persons cannot, without inconvenience, sustain: thus, chylification would appear to require the quiescence of the stomach, and sanguification to be still more incompatible with the act of chymification. If, therefore, the stomach be set to work during the latter stages of digestion, the processes will in weak persons be much disturbed, if not entirely suspended. Certain circumstances cause hunger to return at nearer intervals, by accelerating the nutritive process; while others, by producing an opposite tendency, lengthen such intervals.

1 I state this fact on the authority of Dr. John Badeley, who informed me that he received it from his Lordship's domestic Physician, Dr. Polidori.

117. It is a well-known fact, that if a person be interrupted in his meal for a quarter of an hour, he finds, on resuming it, that his appetite is gone, although he may not have eaten half the quantity which he required. Dr. Wilson Philip explains this circumstance by supposing that the gastric fluid which had accumulated has had time to combine with, and be neutralized by, the food he had taken; but those who believe with me, that a new supply of gastric fluid is furnished, on the contact of every fresh portion of food, must seek for some other explanation. Will not the views which I have offered in the preceding paragraph afford a solution of the problem? viz., that during the suspension of the meal the food had entered upon its ulterior changes, and that the energies of the stomach had therefore declined.

118. The subsidence of appetite, or the feeling of satiety, is not produced by the quantity, but the quality of the food, - the very reverse of what would happen, were the mere volume of the aliment alone necessary to pacify the cravings of the stomach. This is remarkably displayed in the habits of ruminating animals; for in wet and gloomy seasons, when the grass contains a diminished portion of nutritive matter, these animals are never satisfied - they are constantly in the act of grazing; whereas, in hot and dry weather, they consume the greater portion of their time in ruminating, or chewing their cud. I apprehend that this is not to be explained, as M. Majendie believes, by the sensibility of the mucous membrane of the stomach, but is to be solely referred to the fact, that the vital energy is only expended in decomposing such substances as are capable of furnishing chyle. Volume or bulk, however, is a necessary condition of wholesome food: the capacity of our digestive organs sufficiently proves that nature never intended them for the reception of highly-concentrated food.

I some years ago directed considerable attention, in conjunction with some well-known agriculturists, to the nutritive value of different crops, as the food of cattle, and I constructed a logometric scale for the solution of various problems connected with the subject; but I soon found that mere bulk produced a very important influence, and that, to render one species of nutriment equivalent in its value to another, it was necessary to take into consideration the quantity of inert matter which furnished excrement.