The theories so far advanced to explain the genesis of hunger and appetite fall into three main groups, namely, those involving the stimulation of sensory nerves mainly in the digestive tract-a peripheral origin; those involving a direct stimulation of a hypothetical "hunger center" in the brain by the blood, or by some changes in the metabolism in the center itself-a central origin; and lastly those involving a combination of both central and general peripheral factors of hunger-a general sensation.
The theories of a purely peripheral origin of hunger may in turn be grouped under two heads, viz., (i) The stimulation of a strictly local group of sensory nerves (mainly in the stomach), and (2) the stimulation of all afferent nerves by some change in the tissues or in the blood. We shall first outline the theories that account for the hunger sensation by the stimulation of sensory nerves in the digestive tract. There are, to the author's knowledge, at least six such theories, namely: (1) Hunger is due to mechanical stimulation of sensory nerves in the gastric mucosa by mechanical rubbing or pressure from contraction of the stomach. (2) Hunger is due to chemical stimulation of sensory nerves in the gastric mucosa (the acid of the gastric juice, etc.). (3) Hunger is due to the stimulation of sensory nerves in the gastric mucosa by a state of turgescence of the gastric glands. (4) Hunger is due to the stimulation of sensory nerves in the gastric mucosa by some change in the blood due to starvation. (5) Hunger is due to stimulation of sensory nerves in the stomach by the atony and absence of contractions of the empty stomach. (6) Hunger is due to the stimulation of sensory nerves in the wall of the stomach (muscularis or submucosa) especially in the fundus and cardiac regions, by contraction of the empty or partly empty stomach.
Sternberg's literary essays on hunger and appetite contain numerous references to the conceptions of hunger and appetite held by the ancient and mediaeval poets and philosophers; but there appears to be little or nothing specific concerning the hunger and appetite mechanisms in the writings of either Hippocrates or Galen, although the popular view that prolonged starvation or hunger is extremely painful dates back at least to the time of Homer. Hippocrates does remark in one of his famous Aphorisms, that "strong wine cures hunger," but the modus of this cure did not appear to interest him. The theory that the sensation of hunger is due to mechanical stimulation of sensory nerves in the gastric mucosa goes back at least one hundred and fifty years to the great physiologist Haller. On the subject of the immediate cause of hunger, Haller wrote:
Hunger is initiated and intensified by bodily vigor, as in the athlete Milo, in lions, and in all animals possessing great strength by a peculiar strength of the stomach and by physical labor of all kinds, especially at low temperature, since hunger is intensified in cold climates.
Hunger is increased by the presence of intestinal worms, as they consume part of the nutrient juice. It is also augmented by the patency of the pylorus, a condition normally present in voracious animals, and resulting in an almost continuously empty stomach. The sensation of hunger is sometimes even excited by certain acids, such as cream of tartar, lemon juice, and similar substances while regurgitation by putrefying intestinal contents into the stomach depresses and prevents hunger. The numerous kinds of fruits especially enjoyed by the people of the Orient are generally acid.
A greater degree of excitability of the (gastric) nerves also induces greater hunger.
Finally there are imperfectly understood conditions characterized by an inordinate augmentation of hunger, excessive eating, and ingestion of unusual or indigestible substances.
Among animals, those having the shortest span of life, such as the insects, are the most voracious feeders. The caterpillar eats and defecates continuously.
Thus, we see, on the basis of the phenomena of hunger in the entire animal kingdom, the immediate cause of the sensation of hunger is the grinding or rubbing (tritus) of the delicate and vilous folds of the gastric mucosa against each other, through a motion or contraction inherent in the stomach, aided by the diaphragm and the abdominal muscles. These I consider as facts already demonstrated. A famous man1 has suggested that the gastric nerves thus irritated are restored by the congestion of blood in the mucus folds, as occurs in all cases of irritation.
We can show, moreover, that the empty stomach is contracted so that no lumen exists. In a hungry man the stomach is contracted (pinched).
From our knowledge of the intolerable sensation produced by rubbing the exposed nerves in a region where the skin has been lacerated or broken, we are permitted to estimate how acute must be the sensation caused by this friction or stimulation of exposed nerves on the gastric mucosa, as in prolonged hunger. I would not care to face such a Sinister exitus.
These considerations make it clear why a long-continued fast causes only mild hunger in animals like the snake, in whose stomach there is scarcely any grinding action, as it is not sufficiently muscular; besides the strength of the abdominal muscles and the diaphragm is almost nil. They also explain why chickens die sooner from starvation than do dogs, cats, and carnivorous animals in general. Indeed, the grinding action of the stomach in the gallinaceous birds is stronger than in the quadrupeds. In some insects with feeble peristalsis of the alimentary tract there is no hunger in winter, but in the summer the hunger is greater the greater the heat.
So far Haller. It is probable that Haller confuses hunger with appetite when he speaks of acids or bitter fruits augmenting hunger, although if the sensory nerves of the hunger sense are distributed in the gastric mucosa, the possibility of these nerves being stimulated by certain chemicals in the stomach cavity cannot be excluded. The central facts in Haller's conception are the tonicity and contractions of the empty stomach stimulating the hunger nerves in the mucosa by pressure and rubbing.