Physiologists, psychologists, and clinicians who have devoted no special attention to the nature of hunger and appetite appear to accept the common view of the laity that the two sensations are the same in quality, and differ only in intensity. Thus mild hunger is called appetite, while strong appetite is called hunger. If the two sensation complexes are in a linear series and thus involve identical mechanisms, either the term appetite or the term hunger is superfluous, and it would be more correct and less confusing to speak of mild and strong appetite or mild and strong hunger. We do not apply different terms to the sensation of sweetness or of red according to the intensity of the sweet sensation or the saturation of the red color. A sensation remaining identical in quality but differing in intensity should have only one name, and the intensity variation should be denoted by the usual adjectives.
Most authors who have given some special attention to hunger and appetite take the position that the two sensations are fundamentally different, that is, that they differ in quality. This view is supported by Albu, Boas, Boring, Cannon, Carlson, Krehl, Nicolai, Roux, Sternberg, Stiller, and many others. Stiller thinks that appetite is the sensation of hunger plus the memories of taste and smell of foods; hence one may experience hunger without appetite, but not appetite without hunger. But others maintain that one may also experience appetite without the sensation of hunger, as in the case of eating palatable desserts at the end of a full and satisfying meal.
Practically all writers who have recognized the qualitative difference between hunger and appetite state that hunger is primarily an unpleasant and painful sensation and more or less intermittent, while appetite is essentially pleasant in character and without definite periodicity. Hunger, again, is referred to the stomach, while the appetite complex is referred to the mouth and throat.
We have seen that hunger is an uncomfortable pain sensation caused by stomach contractions. Is appetite also a sensation, or is it essentially a memory process ? If appetite is or contains immediate sensory elements, what mechanisms in the mouth and throat initiate the sensory impulses ? In one of his earlier papers Sternberg suggested that appetite is in some way associated with the tonus of the muscles of mastication and deglutition; and that absence of appetite, that is, nausea, is due to atony of these muscles. Later Sternberg suggested that appetite is caused by peristalsis of the esophagus and stomach, while lack of appetite is due to anti-peristalsis in these organs. This appears to us a very superficial suggestion. It is logical, to be sure, but it is contrary to fact. In the very full stomach there is active peristalsis, but that does not necessarily cause appetite. And there is no peristalsis in the esophagus except that induced by swallowing. If esophageal and gastric peristalsis are the stimuli that initiate appetite, this sensation should show a rhythmicity similar to that of hunger.
Sternberg, and after him Vorkastner, and others are guilty of a singular misrepresentation of Pavlov's conception of appetite. These German writers charge Pavlov with saying that "Appetit ist saft"-appetite is gastric juice. We cannot find any such statement in Pavlov's work, and the view is obviously inconsistent with the whole trend of his research on the work of the digestive glands. Pavlov has shown that appetite is a necessary condition for the secretion of appetite gastric juice, but appetite by itself does not induce the secretion, nor does the gastric juice by itself induce appetite, although it may contribute to it by gentle stimulation of the gastric mucosa.
According to Cannon, "appetite is related to previous sensations of the taste and smell of food. These sensory associations determine the appetite for any edible substance, and either this memory or present stimulation can arouse the desire." In brief, appetite is caused either by the immediate taste and smell of palatable food, or induced by the memory processes of such taste and smell sensations.
Some of Boring's subjects described " a kinesthesis in the throat and oral sensation arising from the free flow of saliva, a complex which to them meant desire for food. Here we have a true sensory basis of appetite. The ideation of food is no doubt a usual concomitant, and presumably it often constitutes a desire for food that lacks sensory components entirely." Boring thus includes in appetite the.sensations aroused by increased salivation, and possibly from increased tonus in the muscles of mastication and deglutition, but not the taste and smell sensations, to which Cannon gives such a prominent position. It may be noted, however, that salivation or increased tonus either singly or combined do not by themselves induce the sensation of appetite.
The delineation of the appetite complex and its relation to hunger is a question of analysis of one's own sensation under such experimental control as can be applied. Now, it is obvious that neither the sight, the taste, nor the smell of good food, the memory of these sensations, or salivation and throat kinesthesis can by themselves invariably induce appetite. We all know that the taste or smell of the best of food in the case of severe gastritis induces, not appetite, but nausea. Salivation induced by a drop of vinegar in the mouth does not cause appetite. And in nervous anorexia all of the immediate stimuli and the memory processes mentioned by Cannon and Boring may be present or called forth without producing a desire for food. It is thus clear that a certain sensation complex from the viscera and an approximately normal state of central correlations constitute a necessary background for the development of appetite. Given this background, the central and essential element in appetite is the memory processes of past experience (sight, smell, taste) with palatable foods. These memories are reinforced by present stimulation of these nerves by the food, since everyone knows that appetite, unless intense at the outset, is increased by the very act of eating.
It will be shown that part of this augmentation is due to chemical stimulation of the nerves in the gastric mucosa. The mouth and throat kinesthesis and the sensation due to salivation are, in the author's judgment, accessory elements in appetite analogous to the sensations of weakness and emptiness in hunger.
But how can we account for the desire for food that seems to be inherent in the appetite elements ? We have suggested elsewhere that this urge may be an inherited (partly subconscious) factor (positive chemotropism), fusing with the memory processes of taste and smell of foods as soon as these become a part of the individual's experience. It seems that this urge is present in appetite even when there is no call for food on the part of the empty stomach; hence it is not a vague hunger. Pleasant sensations of different kinds, such as are induced by works of art, music, or the beauties of nature do not seem to contain or arouse analogous desires. In the author's experience the fragrance of the rose in the garden is as pleasing as the fragrance of the roast in the kitchen, but the desire to enjoy the former cannot be compared with the urge to ingest the latter.
It seems to us that the old view that hunger and appetite are different intensities on the same sensation curve is no longer tenable even as a theory. The conception came about through lack or difficulty of analysis of all factors involved, and the tendency to fuse or confuse in consciousness heterogeneous sensations that are usually aroused simultaneously, as is the case with the taste and smell of food. In the normal individual hunger and appetite are usually experienced simultaneously. If only vague or mild hunger is present, the appetite elements occupy the high seat in consciousness; when hunger becomes markedly painful, attention is focused on this element. If we start to masticate palatable food or by means of a stomach tube put a liquid or liquid food directly into the stomach at the height of a period of gastric hunger contractions, the latter are inhibited at once and the unpleasant hunger sensation disappears at the same time, while the pleasant appetite complex is initiated or intensified. In this way we institute a successive contrast between hunger and appetite, so that they can be compared with greater accuracy.
In everyday life of adult persons having access to a liberal food supply the memory elements of appetite are probably a greater factor in the ingestion and digestion of food than hunger, as short intervals between large meals do not permit the development of strong hunger unless the individual is engaged in hard physical work. Under these circumstances appetite and habit supplant hunger as nature's dietary guide. Everyone knows that if the food is made sufficiently palatable we may consume large quantities of it without being hungry or actually needing the food, especially if one has formed the habit of giving marked attention to the pleasures of the table.