Boring, a psychologist, has recently reported a subjective analysis of the character of the hunger sensation, based on the experiences of a great number of persons. He concludes that hunger is a complex of pressure and pain*
Upon a background of dull pressure, which is sometimes recognized definitely as kenisthesis or the equivalent muscular pressure, there is set a dull ache or gnawing pain which characterizes the hunger. Both pain and pressure are referred to the region of the stomach. The pain is noted as fluctuating, as rhythmical, as unstable. Three of the observers described in addition a complex kinesthesis in the throat, and oral sensations arising from the free flow of saliva, a complex which meant for them a desire for food or appetite. Here we have the true sensory basis for appetite.
According to Boring hunger is a twofold experience. It is pressure in its weak form, pain and pressure when intense. This recognition of a pressure or kinesthetic element in the hunger pains is important, and has been made by previous observers.
As the reader will recall, the great physiologist, Haller, stated that the "stomach of a starving man is contracted," and the sensation of hunger is due to the rubbing of the folded mucosa by this contraction. But during the one hundred and fifty years since Haller's Elementa appeared the contracted condition of the stomach in starvation has been questioned, denied, or forgotten. Schiff wrote (in 1867): "The movements of the empty stomach are rare, and much less energetic than during digestion," and as late as the year 1910 Valenti stated that contractions in the empty stomach are rare and feeble. The authors who followed Haller and Weber in accepting the gastric contraction theory of hunger did so essentially without experimental evidence. We take it that all men have observed or experienced rumbling noises (borborygmi) in the abdomen during the period when they feel strong hunger. The noises are due to movements of gas by contraction of some part of the alimentary tract below the esophagus. When the noises are accompanied by the expulsion of gas or air from the stomach into the esophagus the contractions are evidently in the stomach itself, since this occurs in the absence of contractions of the abdominal muscles and the diaphragm. However, in certain "nervous" persons the borborygmi may be very pronounced without being associated with hunger, and it is obvious that these noises may be produced in the large as well as the small intestines. Hertz thinks that these noises arise exclusively from contractions of the small intestines. Nevertheless, the fact that the borborygmi are so frequently associated with hunger does not appear to have been fully appreciated in connection with the genesis of the hunger sensation, until Hertz and Cannon recently-called attention to it.
The view that the empty stomach is atonic and quiescent is the more readily accepted, as contractions would seem to have no useful purpose, except when there is food in the stomach. Now and then later observers (Bettmann, Wolff, His) did record that the stomachs of starving men and other animals are tonically contracted, but the first important study and conclusive demonstration of the motor phenomena of the stomach in starvation we owe to Boldyreff in 1905, although this investigator did not connect the gasjtric contractions with the genesis of the hunger sensation. Working on dogs with inflated balloons in the stomach, Boldyreff found that the stomach of starving dogs exhibits alternate periods of strong contractions and absolute quiescence, at least during the first 3 to 4 days of starvation. The contraction periods last 20 to 30 minutes, the quiescent periods for to 2\ hours. The period of activity is made up of 10 to 20 contractions separated by intervals of 1 to 1 1/2 minutes, beginning with feeble contractions and gradually reaching their maximum strength at the end of the period. During these periods of gastric motor activity there were also contractions in the intestines. Boldyreff states that the contractions of the empty stomach are stronger than the gastric peristalsis during digestion. There were no contractions in the empty stomach during the periods when the gland secreted gastric juice copiously. Hertz (1911) gave a different interpretation to these contractions, namely, that they give rise to the hunger sensation.
These are fundamental and important facts. Boldyreff did not think that these gastric contractions gave rise to the sensation of hunger, mainly because they diminished in strength with the length of starvation. He suggests the possibility that the state of hunger in the brain initiates the gastric and intestinal contractions via the motor nerve fibers in the vagi. It seems hardly necessary to point out that the genetic relations of the contractions of the empty stomach to the hunger sensations cannot be established with certainty on experimental animals below man, because of the difficulty in determining the kind of sensation experienced by the animal.
In 1910 Hudek'and Stigler reported that the stomach empties faster when the food is eaten with hunger than when eaten without hunger. Cannon cites this fact as evidence that the stomach is in greater tonus in the hunger state. It is obvious, however, that an equally important factor is the greater rate of digestion owing to the appetite gastric juice. The difference in the time of emptying of the stomach observed by Hudek and Stigler may be due to this factor alone.
In 1911 Cannon and Washburn by experiments on man (Washburn) proved that the periods of contractions in the empty stomach are synchronous with the periods of hunger sensation, and that each separate contraction is synchronous with a hunger pang. In their experiments a small balloon was swallowed into the stomach and the stomach contractions were recorded graphically parallel with a signal showing when the subject felt the hunger. These observers also obtained evidence of contractions in the lower third of the esophagus, synchronous with the gastric contractions, and conclude that the esophagus plays a part in the genesis of hunger. While noting that the hunger sensation tends to lag behind the gastric contraction both at its beginning and its cessation, thejr experiments, nevertheless, do not prove that the gastric and esophagus contractions cause the sensation of hunger by stimulation of sensory nerves, but they assume this to be the mechanism. Nor do they indicate, on this assumption, whether the nerves stimulated are those in the gastric mucosa, or in the muscular wall of the stomach. This demonstration of the synchrony of the gastric contractions and the subjective feeling of hunger is an important step, but it does not inform us which is the cause and which the effect. In other words, adherents of central or hunger-center theory may still maintain that the gastric contractions are initiated by motor discharges via the vagi nerves under the influence of a periodic activity in the hunger center in the brain.
A year later (1912) Carlson and his pupils, using essentially the methods of Morat, BoldyrefT, Cannon, and Washburn, demonstrated on man and experimental animals that a certain type of contractions in the empty or nearly empty stomach gives rise to the sensation of hunger by stimulation of sensory nerves, not in the gastric mucosa, but in the submucosa or muscularis. These hunger contractions of the empty stomach are primarily initiated in the stomach itself and are thus independent of motor impulses from the brain or the spinal cord. The peripheral or gastric origin of the essential element in the sensation of hunger was thus finally established.