The weakness accompanying hunger is evidently of complex origin, or partly due to sensory impulses from the digestive tract, and partly to relative exhaustion in the tissues. Voit thought that the feeling of weakness is a general sensation in some way caused by lack of nutrient materials in the blood. That in moderate hunger only the first or reflex factor is involved is evident from the fact that this weakness is abolished by taking food into the stomach, before there is any digestion and absorption of the food material into the blood. After several days of starvation, the feeling of weakness does not all disappear immediately following a meal. Hence we must here be dealing also with the factor of replenishing the tissues. How does the hunger state of the stomach and intestines induce the feeling of weakness? We have seen that this hunger state means strong tonus and contractions of the empty stomach and intestines. It probably also involves a greater degree of tonus of the abdominal muscles in the maintenance of the intraabdominal pressure. It is not clear, however, why tonus and contractions in the empty stomach should in some people produce the feeling of weakness.
We are not convinced that the vasomotor disturbances caused by the hunger contractions are great enough to produce the feeling of weakness. There are probably other reflex factors at present unknown. At the same time, the degree of weakness caused by identical peripheral factors evidently depends on the stability of the central nervous organization, for in disease (hysteria, neurasthenia, etc.) a degree of hunger which, in a normal person, would not induce the feeling of weakness may cause extreme prostration.
Strong sensation of hunger is usually accompanied by a peculiar feeling of "emptiness" in the entire abdominal region. This feeling is continuous, not intermittent like the pangs of hunger. The "emptiness" feeling does not disappear entirely during the relative quiescences of the empty stomach between the hunger periods.
This feeling is probably more complex in its origin than the hunger pangs. It is well known that a very complete evacuation of the large intestines, as by enemas or purgatives, induces a feeling of emptiness. The feeling may be partly abolished by moderate but steady pressure on the abdominal wall. It is probable that the increased tonus of the abdominal muscles, in consequence of the empty state of stomach and intestines, contributes to the feeling in some way. It is well known that the intra-abdominal pressure remains nearly constant under varying degrees of fulness and emptiness of the digestive tract.
Since in starvation there is persistent hypertonus of the abdominal muscles, we question whether any part of the feeling of emptiness originates in the stomach itself. It is not so difficult to understand how a hypertonic and rhythmically contracting empty stomach may give rise to sensations of tension, pressure, and gnawing pain. But how can it cause the sensation of emptiness, unless this feeling is merely the negative of the sensation of fulness ? 1 On the other hand, if the tonus of the abdominal muscles does not suffice to maintain the normal intra-abdominal pressure when stomach and intestines are relatively empty and strongly contracted in hunger, the tension on all the visceral organs would be diminished, and this in turn would alter the pressure relation to the peritoneum and the mesentery. If this is a factor in the origin of the feeling of abdominal "emptiness" in hunger, the sensation should be diminished in man by lying down, in comparison with that felt when standing. The writer cannot test this hypothesis on himself, as the feeling of "emptiness" is not a prominent part of his hunger complex.
The author experiences slight headache and a suggestion of nausea only after 4 or 5 days' starvation, but there is no doubt that a few apparently normal people experience both, or more particularly the headache, during the moderate hunger that comes on 4 to 6 hours after eating. Some of Boring's subjects had difficulty in distinguishing between hunger and nausea in their own consciousness. In such cases there can be no starvation change in the blood and tissues. And the fact that headache as well as nausea is greatly relieved at once by taking food into the stomach seems to show that they are essentially of gastric or reflex origin. It is well known that both headache and nausea can be produced by disturbances in circulation, and that nausea can be caused by stimulation of nerves in the stomach mucosa (normal stimulation of hyperexcitable nerves, or excessive irritation of the normal nerves). Possibly the strong hunger contractions of the empty stomach cause sufficient stimulation of the gastric nerves to induce both headache and nausea in certain individuals. This can be determined experimentally on such persons. Unfortunately, the author does not himself experience these accessory hunger phenomena, nor has he been able to secure as subject a person with these hunger symptoms sufficiently marked to make it worth while investigating. In prolonged starvation there is a contributory factor, namely, a greater degree of increased excitability both of the gastric nerves and of the central nervous system.
So far as established facts permit conclusions or point the way, the accessory hunger phenomena, excepting the exhaustion fatigue of prolonged starvation, are caused reflexly by the hunger tonus and the hunger contractions of the empty stomach. The degree with which they are manifested in hunger depends on the intensity of the hunger contractions, the excitability of the sensory nerves of the stomach, and on the relative stability of the central nervous organization of the individual person.