Substances like strong alcohol or acid (5 to 20 per cent HCl), mustard, pepper, etc., introduced by tube directly into the stomach in sufficient quantities, cause varying degrees of pain, accompanied at first with a peculiar feeling of warmth in the stomach. In concentrations sufficient to cause pain, all chemicals evidently injure the mucosa and its nerve-endings, as shown by the development of gastritis. Boring states that the pain induced by 2 to 10 per cent HCl in the stomach is identical with the pangs of hunger, and surmises that the acid in the stomach causes gastric hunger contractions. This surmise is contrary to all experimental facts, at least on normal men and dogs.
When these and other chemical substances are put into the stomach too dilute to cause ache, pain, or discomfort, their contact with the normal gastric mucosa still affects consciousness, but in a manner the very opposite to that of pain. The sensation produced by this gentle chemical stimulation of the mucosa has a character of its own that resembles if it is not identical with appetite. We cannot understand how Hertz and Schmidt could have reached the conclusion, on the basis of experiments on man, that the normal mucosa is insensitive to chemical stimuli, including 2 per cent mineral acids (HCl).
Pavlov concludes that "the tactile sensation of the stomach at the moment of entry of food is capable of awakening or increasing appetite." In the experiments cited by Pavlov in support of this conclusion the stimulation of the nerve-endings in the mouth and in the esophagus was not excluded.
The author, working on himself, soon recognized that moderate chemical stimulation of the nerve-endings in the gastric mucosa modified the flow of consciousness, although this modification did not consist in the sensation of hunger. This can readily be experienced by anyone who is sufficiently interested to try, by introducing moderately cold water, beer, wine, weak acids (0.5 to 2.0 per cent HCl), weak alcohol, or carbonated drinks through a tube into the stomach so as to avoid stimulation of nerve-endings in the mouth and esophagus. The sensation produced by these substances in the stomach is rather transitory, but may persist for several minutes. With the exception of cold water, which is also felt as cold, these various substances give rise to a characteristic sensation which fuses with, or cannot be distinguished from appetite. It is like the sensation of increased appetite experienced by most people at the beginning of a meal, after eating a few morsels of palatable food. The sensation is pleasant, and turns the attention toward food and eating.
We are accustomed to think that the substances named above affect consciousness solely through stimulation of nerve-endings in the mouth. This view is no longer tenable. By introducing these substances through the stomach tube at the height of a gastric hunger contraction, one actually experiences a successive contrast of the sensations of hunger and appetite, as these substances temporarily inhibit the hunger contractions in stimulating the gastric mucosa. From the first it was clear that, when beer or cold or hot water was first introduced into the stomach during a vigorous hunger contraction, the sensation resulting was the exact opposite of that caused by the hunger contraction. In place of an unpleasant tense sensation, associated with restlessness, the sensation caused by these different stimuli is one of relief. A pleasant, tingling sensation is felt in the stomach. One feels perfectly at ease, but the thoughts tend to revert to the dinner table. At first, we were not able to say just what this sensation was like, although it was a familiar one. After paying close attention to the sensation experienced at meals just after a few mouthfuls of good food or drink have been swallowed, we became convinced that the two sensations are very much alike, if not identical.
How do we know that the sensation temporarily produced by the above-named substances in the stomach is directly due to stimulation of nerve-endings in the gastric mucosa? Since the introduction of these substances in the stomach inhibits the gastric tonus and the gastric hunger contractions, may not the sensation be one of negative character, so to speak, that is, diminution or absence of hunger? We are in position to answer this question definitely in the negative. In the first place, the sudden and spontaneous relaxation of the stomach at the end of a period of gastric hunger contractions is accompanied by a characteristic sense of relief and disappearance of a certain tension or unpleasant mental stress, but this sensation complex has not the positive character that directs attention to food and eating. It is essentially relief from pain. Secondly, putting these substances into a stomach which is quiescent and very greatly relaxed, nevertheless, inaugurates this temporary appetite or appetite-like sensation. Hence we conclude that it is directly induced by stimulation of certain nerve-endings in the gastric mucosa itself. Of course, if the nerve-endings in the gastric mucosa are thus stimulated at the time the muscularis is in strong tonus and hunger contractions, the appetitelike sensation is fused with that of relief from the pangs of hunger.
It is significant that normal human gastric juice having full acid strength (0.45 to 0.50 per cent free HCl) is capable of inducing this sensation from the stomach. This has been verified repeatedly by introducing 50 c.c. of appetite gastric juice into Mr. V. through the stomach tube. Gastric juice of weaker acidity (0.20 per cent) does not have this effect. As this full-strength gastric juice is rapidly secreted in the stomach at the beginning of eating, it is probably a factor in the augmentation of appetite by the very act of eating. It may be pointed out that the foregoing facts permit us to see a suggestion of truth in Beaumont's theory that tumescence of the gastric glands is the cause of hunger. 'In the first place, Beaumont, in common with most physiologists, did not i clearly distinguish between hunger and appetite, but used the two terms interchangeably. If we, then, substitute appetite for hunger, we see that with the stomach normal, the appetite sensation may be actually initiated or augmented by gastric juice, not through mechanical pressure of the juice in the ducts, but by acid stimulation of nerve-endings in the gastric mucosa.
It need scarcely be pointed out that when foods or liquids are taken into the mouth and swallowed in the normal way their main influence on appetite is by way of nerve-endings in the mouth. In fact, this influence is so predominant that only by excluding it are we able clearly to distinguish the gastric factor. The memory factor in appetite is therefore pre-eminently gustatory and olfactory.