As experienced by man, the hunger urge is a more or less uncomfortable feeling of tension or pressure and pain referred to the region of the stomach. In normal persons the hunger must become exceptionally strong to be markedly painful. Ordinarily the feeling is one of somewhat uncomfortable tension, accompanied by a feeling of "emptiness" in the epigastric region.
Another characteristic element in the hunger feeling as known in man is its periodicity or intermittency, even when the stomach continues empty and the physical activity of the individual proceeds without interruption. The significance of this attribute has been especially emphasized by Bardier, Sternberg, Polimanti, Cannon and Washburn. It probably applies to the hunger sensation in most mammals and birds. In the lower animals, so far investigated, this periodicity is of a different type, or is possibly lacking. Some persons appear to experience a certain feeling or sensation in the esophagus, the throat, and the muscles of mastication synchronously with the gastric hunger. The writer has never experienced these esophagus and throat elements. From all accounts, they differ from the gastric sensation in not being uncomfortable or painful. In the species of birds having a region of the esophagus dilated into a crop, the hunger sensation probably has its origin in the crop rather than in the muscular stomach or gizzard.
The epigastric sensation of varying degrees of pain is, however, the one indispensable element in hunger. But frequently certain accessory phenomena are present. The most common of these is a feeling of general lassitude or weakness. Headache, nausea, nervous irritability, vaso-motor instability, and even fainting may appear as part of the hunger complex. Strictly speaking, a certain degree of nervous hyperexcitability is a necessary effect of hunger of even moderate intensity, and should therefore not be called an accessory phenomenon. But in normal persons with stable nervous organization, strong hunger may be present without any feeling of weakness, headache, or obvious manifestations of nervousness. In some individuals, on the other hand, the feeling of weakness, headache, and general restlessness may be so marked as to crowd out of consciousness the central factor of hunger, the gastric hunger pangs.
In man the genesis of the hunger sensation requires a stomach empty or nearly empty of food. Adult persons eating three to five large meals per day probably seldom experience- hunger unless engaged in severe physical labor or exposed to intense cold. In such individuals the nervous impulses from the stomach that give rise to the hunger pangs do not become sufficiently intense to affect consciousness.
We shall show later that the genesis of the hunger pangs is due to certain contractions and tonus states in the stomach, the afferent nervous impulses thus initiated affecting certain parts of the brain. Hunger as known in man thus requires a nervous system, a muscular digestive tract, and an afferent or sensory pathway connecting the two. These anatomical conditions are common to all vertebrates, and to the various invertebrate groups down to and including the coelenterates.
The influence of the hunger state on the behavior is essentially the same in all these animals. A fundamental characteristic is increased nervous excitability and restlessness. The restlessness is probably not primarily due to the consciousness of the hunger feeling, since it is in evidence in animals deprived of their cerebrum (dogs, birds). The strong hunger urge evidently inhibits fear, as the starving animal becomes more bold and ferocious. It is generally held that the state of hunger in man tends to produce a cantankerous or unsocial disposition. On the other hand, prolonged fasting by the religious devotee is supposed to make him more worthy or fit to commune with the gods.
In the normal man, and probably in the carnivorous animals, an empty or a nearly empty stomach is a requisite for the appearance of the hunger feeling. This is evidently not the case in the ruminating animals and in the herbivora in general, for in these animals the stomach is never empty-not even after days of starvation. Birds, also, feed more or less continuously, even though the crop or the stomach is quite filled with food. Either these animals do feel hunger on a partially filled stomach, or else they eat, not because of feeling the hunger urge, but because of appetite.
On the other hand, hunger may be apparently absent in some animals, even though the stomach is completely empty of food for weeks or months. It is generally accepted as a fact that the Rhine and the Pacific species of salmon do not feed after entering the rivers to spawn, although the fish is doing great muscular work in going hundreds of miles against river currents, in ascending falls, and in fighting rivals. Voit thought that under these conditions the fish cannot be feeling hunger. When in captivity, certain animals may refuse food even to the point of starving to death in the presence of plenty. The call of the empty stomach, if sufficiently strong, wakes up the sleeping child, the sleeping man, or the sleeping dog, but the empty stomach does not appear to disturb the hibernating animal. " The caterpillar/' says the great physiologist Haller, "does nothing but eat and defecate." After the caterpillar has turned into a butterfly h£ may never feed again, particularly if his span of life is a short one, despite great physical exertion in flight and reproduction. It does not appear that this diminution or absence of feeding in many insects after the final metamorphosis is associated with atrophy or absence of the alimentary tract. These special conditions obviously involve changes in the brain processes, in the stomach, or in the nervous connections between the brain and the stomach, which must be cleared up by new lines of work; but they do not overthrow the foregoing view of the rdle of the hunger urge in feeding, or the rdle of the empty stomach in the genesis of this urge.
Except in the peculiar and special cases referred to above there is evidence that the intensity or persistence of the hunger urge runs parallel with the degree of activity and the rate of metabolism in the normal individual. Hunger is thus more marked in the young and growing than in the aged and inactive individual. There are indications in mammals that hunger may be experienced even before birth. In warm-blooded animals hunger is augmented by external cold, and depressed by external heat. The reverse is probably true in the cold-blooded animals, but this point has not been experimentally determined. These relations do not obtain in various conditions of disease.