The term "appetite" in relation to dietetics usually means a pleasurable desire for food or drink, whereas hunger and thirst imply a craving for food and drink respectively, which has become disagreeable or positively painful. There is, however, no distinct line of demarcation to be drawn between these terms.

The appetite for food is a most capricious sensation, subject to all manner of disturbing influences. It is to some extent apparently under control of the will, in that it can be trained to recur at certain intervals before taking food. In a normal state it is, therefore, rhythmical, and it may then be taken as an index of the need of food, but when it becomes abnormal it is a very unreliable guide.

"As a general rule, though by no means without exception, substances pleasing to the palate are useful and not injurious to the organism" (Brunton).

The appetite often appears with great suddenness, either independently or as the result of directing the attention to matters of food and eating. It may depart as suddenly, even without gratification, or it may vanish after the first few mouthfuls of food are eaten, although it was apparently vigorous a moment before.

The appetite is aroused by a variety of circumstances and conditions, both physical and mental. Such are the smell, taste, and sight of food, good hygienic surroundings, exercise, bathing, cold or stimulating air, agreeable companionship, pleasurable mental emotions, and the proper preparation and serving of food. It may be stimulated by bitters, condiments, such salt foods as caviare or herring, and in some cases by alcohol. Wine drunk between meals is apt to spoil the appetite, but taken in moderation with meals it may increase it. A substance known as orexine, in the form either of a hydrochlorate, tannate, or simple basic condition, has been recommended as having the special function of exciting the appetite in convalescents. It does not, however, give uniform results, and I have seen little or no benefit from its use.

To obtain the most complete satisfaction from the sense of taste one should swallow the food, and not merely take it into the mouth.

The appetite is usually somewhat more keen in winter than in summer, but many persons observe no difference. It is depressed or destroyed by mental emotion, especially grief, anxiety, and worry; by the sight, smell, or taste of ill-prepared or improperly cooked or badly served food; foul air and poor hygienic surroundings; fatigue and exhaustion; many diseases, more particularly febrile diseases and most gastric disorders; nausea; the abuse of strong condiments, and of many drugs, notably opium and those which, like potassium iodide, produce a continual offensive taste in the mouth; the abuse of alcohol; eating irregularly and at too short intervals. In old age the appetite, especially for meats, usually becomes less keen, and the absence of teeth contributes to the loss of desire for such food. A voracious appetite sometimes occurs in children.

Bulimia, which means excessive craving for food, is by no means an indication of vigour, and is often due to an irritable condition of the nerves of the stomach, and may be brought about by eating at irregular intervals, which results in disturbance of the gastric secretion. Such children are usually thin, and are encouraged by ill-advised parents or attendants to gorge themselves with food which they do not digest.

The appetite is very dependent upon habit and upon the usual order and arrangement of the meal. An attempt to eat a lump of butter alone usually fails, but it is easily consumed if spread upon bread. Reversing the customary order of the different foods served at a dinner usually produces disgust, and may even excite nausea.

Pawlow has shown by a series of elaborate digestion experiments, made at the St. Petersburg Institute of Physiology, that there are two "tides " of gastric-juice secretion. The first, "appetite juice," is induced by hunger and the pleasurable sight or smell of food, or the sounds associated with its preparation, and is psychic; this secretion is abundant and actively digestant. The second is excited by the mechanical and chemical action of food in the stomach. Of the two the former often proves the more important, for food which is unappetising, or food which for any reason is eaten without relish or while the mind is strained in other channels, may remain for hours undigested. Hence the failure oftentimes to secure hydrochloric acid after an unappetising "test meal" of bread and water.