Erasmus Darwin thought that the hunger pain is due to the atonicity and absence of contractions in the empty stomach. The view that the empty stomach of normal individuals is atonic and quiescent has persisted in physiological and medical literature to this day, despite both early and recent evidence to the contrary.
Johannes Miiller, one of the fathers of modern biology, states that hunger is a kind of negative sensation-or simply due to the absence of the positive sensation generated in the stomach during digestion. If this is the case, an individual should always feel hunger after excision of the stomach or after section of both vagi nerves. Miiller notes, however, that the stomach of animals after prolonged fasting or after death from starvation appears very much contracted.
1 The physiologist Senac.
Weber considered it probable that "sudden and strong contraction of the empty stomach, completely obliterating the gastric cavity, gives rise to a part of the sensation we call hunger." As analogies to the hunger pangs he refers to the labor pains (uterine contraction), the pangs from the large intestine in tenesmus, and from the small intestine in cases of colic. Weber does not explain how the gastric contractions stimulate the hunger nerves. This general view that the hunger is caused by contractions of the empty stomach has been accepted by a number of physiologists and clinicians (Vierordt, Hertz, Knapp, and Sternberg), and finally demonstrated by Cannon and Washburn, and Carlson. Voit, Albu, Stiller, Nicolai, and others assume a gastric genesis of hunger without going into the question of how the stimulation is brought about. Sternberg has designated hunger as "Pruritus stomachi," or a tickling sensation, similar to that evoked from mechanical stimulation of certain cutaneous areas. He also suggests that appetite is in some way correlated with the peristalsis of the esophagus and stomach and that absence of appetite or nausea is similarly associated with antiperistalsis. Sternberg's papers on the subject of hunger and appetite contain no original observations, and one meets with a number of contradictions and far-fetched analogies that prove nothing. Thus Sternberg says that "the empty stomach is anatomically an atonic folded tube." Now,, an atonic stomach cannot give rise to hunger by contraction or peristalsis, for the simple reason that these are not present, except when there is a certain degree of tonus.
The turgescence theory of hunger as formulated by Beaumont is untenable, for the reason that there is no actual accumulation of gastric juice in the crypts of the glands in the empty stomach to stimulate the gastric nerves by distension. Moreover, there may be a continuous secretion of gastric juice during a hunger period. Yet the theory is accepted, at least in part, by some recent workers (Luciani, Valenti).
The theory that the sensation of hunger is a mere negative phenomenon, or the absence of the positive sensation accompanying the filled stomach, has not received much attention. If we consult our own experience, the hunger urge appears to be a sensation as positive as pain. We also know that the mere emptiness of the stomach does not initiate the sensation.
The view that hunger is due to the chemical stimulation of nerves in the gastric mucosa appears also to lead back at least to Haller. Soemmering ascribes the hunger pains of fasting to the action of gastric juice on the mucosa nerves. Bostock accepts the theory and credits it to "the chemical physiologists" (the itra-chemical school?). Cannon thinks it is based mainly on clinical evidence in cases of so-called gastric hyperacidity and hypersecretion. Pavlov appears to accept it, when he cites his own experience of hunger being initiated by a small quantity of wine passed into the stomach. It is well established, however, that we may feel hunger when the stomach is completely empty of gastric juice and other substances that may be capable of stimulating the nerves in the mucosa. -Some mucus is always present in the stomach, but there is no evidence that this can act as chemical stimulus. It is also known that in cases of complete absence of hydrochloric acid in the gastric juice (achylia gastrica) hunger may be present.
Magendie thought that hunger is strictly of central origin, not even directly due to depletion of the blood or the tissues, since both the gastric sensation and the general feeling of weakness may pass away without the individual partaking of food. He denied tonic or periodic contractions of the empty stomach, at least during the first 3 to 5 days of fasting, and quotes only to reject mechanical stimulation of the stomach walls, traction on the liver by the diaphragm, fatigue (atony) of the gastric musculature, bile and gastric juices in the stomach as causes of hunger; but he does not deny that sensory impulses for the organs in general may be contributing factors. Tidemann, Schiff, Ewald, Wundt, Milne Edwards, and others adhere in the main to this theory but assume that the hunger center is stimulated by a starvation state of the blood. Many authors have pointed out the analogous condition in the case of the respiratory center being stimulated by the venous condition of the blood.
The main objection urged against this theory is its failure to explain, (i) the reference of the hunger sensation to the stomach; (2) the fact that hunger may be temporarily abolished by the eating of indigestible materials, and (3) the periodicity of the hunger sensation.
The main arguments urged by exponents of the central theories against the gastric origin of hunger are: (1) Man and animals will eat after excision of the stomach, or after section of the sensory nerves to the stomach. (2) Hunger may be present even when the stomach is partly filled with food. (3) Hunger may be appeased by feeding per rectum as well as by intravenous injections of foodstuffs.
As formulated by Bardier, this theory assumes that the hunger center in the brain is stimulated directly by some change in the blood, and indirectly by afferent nervous impulses from all organs of the body, the stimulation of these afferent nerves being also due to some change in the blood induced by the state of hunger. With certain minor modifications and additions, this view is accepted by Longet, Beaunis, Roux, M. Foster, J. L. Miiller, Schlessinger, and others. According to Roux the gastric moiety of the peripheral part of the hunger sense constitutes appetite, that, is, appetite is due to the stimulation of sensory nerves in the stomach by the starvation changes in the blood. Miiller, writing in 1915, cannot very well deny the gastric hunger contractions, but he assumes that these contractions are caused through motor impulses in the vagi nerves by stimulation of the hunger center in the brain by the blood, although this possibility was disproved in 1913 by Carlson. That the stimulation, central and peripheral, is brought about by some changes of the blood in starvation, is the main element in the theory. The analogy of this mechanism to that of the genesis of thirst from the osmotic concentration of the blood has been frequently pointed out in support of the theory. DuBois-Reymond (quoted from Nicolai) postulated a vagus hunger (gastric origin) and a tissue hunger (the feeling of weakness). Turro advances a" trophic reflex mechanism" of hunger. Lack of nutrient material stimulates nerve-endings in all organs. The impulses pass to a hypothetical trophic center in the basal ganglia of the brain, and these lower centers, in turn, affect consciousness. He intimates that this "trophic hunger mechanism" is so perfectly adjusted by inheritance that the animal on seeing food knows instinctively, as it were, just how much salt, water, fat, starch, and protein to ingest.
The objections raised by various authors against this theory are: (i) Hunger may set in before intestinal absorption is completed, and therefore before there can be any starvation change in the blood. (2) There are not very marked chemical changes in the blood even in prolonged starvation. (3) Hunger is abolished temporarily by the eating of indigestible matter, and by taking food hunger ceases before any of the food is digested and absorbed. (4) Hunger is usually more or less periodic with sudden onsets and endings, while changes in the composition of the blood from starvation are more likely to be gradual and continuous. (5) In very prolonged starvation hunger does not, at least in man, increase in intensity with the supposed depletion of the blood, and in fevers hunger may be absent despite prolonged starvation with increased metabolism.
That the sensation of hunger is not an evidence of the immediate need of food because of starvation changes in the blood was pointed out by Voit in the following way. The people of Ireland, used to voluminous rations of potatoes, complain of starvation and hunger when given even greater food values, but in smaller bulk. The same is true of the bread-eating peasants of Bavaria when put on a diet of meat.