Why does this feeling of uncomfortable tension or hunger pang in the stomach induce us to take food ? The obvious reply is that we know that eating will abolish the pangs. That is, it is a matter of individual experience. This answer does not seem adequate in the case of the newborn animal. Even if the animal experienced the sensation of hunger before birth, he cannot have experienced what effect the ingestion of food has on this sensation. What induces the newborn animal to take his first meal ? Is it a matter of inherited reflexes or "instinct" ? Gemelli and others refer to hunger as an "instinct." The animal emerges from the egg or uterus provided with all the essential reflexes, the working order of most of these probably already tried out before entering upon the new mode of living. The feeding reflexes are there; so are the defensive ones in the way of rejection of unpalatable or really injurious substances once in the mouth. We have seen that the hunger state of the stomach augments the reflex excitability. This leads to greater degree of moving about on the part of the newborn animal capable of locomotion, and to greater activity of the feeding reflexes in all. This the writer has observed in decerebrated pigeons kept in good condition for months after the operation. When the crop becomes empty this bird not only becomes restless and keeps walking about incessantly, but picks at the floor, the walls of the cage, or the empty air. If it happens to pick at food before it, there is no indication of " recognition " of it as food. Nor does it open its beak in going through the motion of picking. The significant fact in this connection is the inducing of the picking reflex by the hunger state.
In the newborn everything within reach goes into the mouth to be rejected or swallowed according to its chemical character or physical consistency. It would, then, seem that the newborn connects up the gastric hunger urge with the processes of feeding as a matter of individual experience by the method of trial and error. In man there is conscious direction on the part of the mother. Conscious direction as well as the factor of imitation probably plays a r61e in newly hatched chickens "learning" to feed. In the young birds that secure their food by thrusting their beak and head down the throat of the mother, maternal direction is also probably the initial factor. The hunger sensation induces the movements of sucking in the newborn mammal, while in the newly hatched and immature bird it causes the beak to be set wide open. Such minor differences are obviously a matter of inherited reflexes, and we need not fall back on "instincts" to account for them.
On the foregoing hypothesis the connection between the primary sensation of hunger and the processes of feeding is a matter of individual experience by the method of "trial and error." The fundamental factors are the augmentation of all reflexes by the gastric hunger state, the removal of the hunger pangs, and the production of the opposite sensation of satiety by sucking, mastication, and ingestion of food. The theory demands the presence of memory, otherwise each feeding act or feeding period becomes a matter of trial and error.
We may ask whether this gap between the pure sensation of hunger and the ingestion of food in the newborn is not in reality bridged by the factor of appetite. There is as yet great uncertainty and confusion in regard to the elements of appetite, and the relation of appetite to hunger. Many physiologists appear to accept the view that appetite and hunger involve identical mechanisms and differ only in degree of intensity. That is, a mild state of hunger is called appetite, and a strong appetite is called hunger. There are others who maintain that hunger and appetite are different, both in the quality of the sensation and in the mechanisms involved in the genesis of the sensation. The latter view appears to make the nearest approach to actual conditions.
Appetite, as we know it, cannot be separated from our memory . of past experience with food, that is, the taste, smell, and appearance of food. In fact, it appears to be essentially pleasant memory processes of these past experiences, and the "urge" in appetite " may be only a special case of the general desire for pleasure. If this is the case, there can be no urge for food in the absence of past experience with food on the part of the individual, and as this is lacking in the newborn, the appetite urge must also be wanting. Hence it is not the factor that guides the newborn individual for the first time to abolish the pangs of hunger by ingestion of food. However, it is conceivable that appetite contains an elemental urge for food as an inherited mechanism and thus not dependent on individual experience, and that when the individual has such experience with food, memory processes of this experience fuse with or overshadow the inheritance factor, so that the two cannot be dissociated in consciousness. This inherited appetite urge, if it exists at all, is probably essentially a positive chemotropism (smell, taste), although birds hatched sufficiently mature to seek their own food probably select or seek the food by vision rather than by smell.
This positive chemotropism for food may in the newborn of the higher animals involve an element of pleasure. It is clear that such an inherited positive chemotropism augmenting the motility and guiding the movements, plus the inherited reflexes of taking everything within reach into the mouth, will lead to food ingestion in the newborn, even in the absence of hunger. But if the latter is present the food ingestion completes the experience of removal of the unpleasant hunger pangs by feeding. Loeb and others have referred to a positive chemotropism as a factor in the finding and selection of food, especially in the lower animals, but we do not know whether in the species thus controlled a state of hunger means an increased excitability and augmented motor response to the specific chemical stimuli.
It would thus seem that the first ingestion of food on the part of the newborn or newly hatched animal that feeds unaided by the parents can be accounted for by either of the theories outlined above. Given the hunger pangs with .its effects in the way of increased motility and reflex excitability and the inherited reflexes of putting everything within reach into the mouth, plus the reflexes causing rejection of injurious or "disagreeable" material, the ingestion of food becomes only a question of it being within reach, and the experience of removing the uncomfortable pangs of hunger by feeding is quickly established. On the other hand, given an inherited appetite urge or positive chemotropism, so that certain olfactory and gustatory stimuli initiate and direct the reflexes and possibly produce a sensation of pleasure, ingestion of food is equally inevitable.
The first experience of feeding once gained, the individual will feed again because of appetite, that is, the pleasure in the tasting and smelling of food, and the pleasure in the sensation of satiety, or because of hunger and the experience that feeding removes the hunger pains.
In the discussion so far we have used the term "food" mostly in the restricted sense of organic food substances. In a wider sense the term "food" includes all materials necessary for the continuance of the life of the animal. The sensations induced by the lack of water (thirst) and By impairment of the external processes of respiration (dyspnea) have nothing in common with the sensation of hunger (due to lack of pabulum in the stomach), either in their genesis or in their character, except that all three sensation complexes are more or less uncomfortable or painful. But we may speak of the sensation of thirst for water as analogous to the sensation of hunger for food. There is, however, no appetite for water analogous to the appetite for food. This is probably due to the absence of taste qualities in pure water, and hence to absence of memory representations of taste. In the case of beer or other artificially flavored drinks that may be taken to satisfy thirst, taste qualities are present, and persons may develop an appetite for these drinks in connection with as well as in the absence of actual thirst, just as we may have appetite for certain foods with or without actual hunger sensation.