All acids, or liquids containing acids, including normal human gastric juice, cause inhibition of the movements and the tonus of the empty stomach when introduced directly into the stomach cavity. No acid has been tested in stronger concentration than 0.5 per cent. The duration of the inhibition is on the whole directly proportional to the concentration and the total quantity of acid introduced; 200 c.c. of 0.5 per cent of HCl will usually inhibit for a period of 25 to 30 minutes only.
This inhibition by acids can be made evident during all stages of Activity of the empty stomach. If the acid is introduced during relative quiescence of the stomach, the appearance of the next period of hunger contractions is delayed; if introduced during the active contractions, these are abolished or depressed.
The duration of the acid inhibition is probably determined by three factors, namely, (1) passing of the acid into the duodenum, (2) fixation and neutralization of the acid of the mucous gastric secretion, (3) neutralization by bile and intestinal juice which at times pass into the stomach through the dilated pylorus.
While it is a striking fact that gastric juice of full normal j acidity (0.48 to 0.53 per cent) and other acid solutions inhibit the J hunger contractions, it does not follow that a neutral or alkaline reaction in the gastric cavity is a prerequisite for these contractions. During the strong contractions the stomach secretes a juice rich in mucin and combined HCl, but usually containing some free HCl. After the introduction of acids the contractions reappear before all the acid has passed out of the stomach or has been completely neutralized. And in case Mr. V. chews palatable food during a strong hunger period, the hunger contractions reappear before there is complete cessation of the psychic secretion of gastric juice. In other words, the hunger contractions are not inhibited by weak concentrations of acids in the stomach. "A neutral or alkaline reaction of the mucosa is not necessary for these contractions. If the food is sufficiently palatable and the mastication is continued long enough, the inhibition produced reflexly from the mouth fuses with the acid inhibition from the stomach. If the food is not especially palatable or the mastication period brief, the contractions may resume on cessation of the chewing and then again be inhibited for a time during the period of most rapid secretion of the appetite gastric juice.
The degree of inhibition produced by normal gastric juice is the same as that caused by an equal quantity of hydrochloric acid of a concentration equal to the free acidity of the gastric juice. It would thus seem that the hydrochloric acid in the gastric juice constitutes the stimulus that leads to the inhibition.
This acid inhibition of the hunger contractions is of peculiar interest in connection with the neuromuscular mechanisms of these hunger movements and,the gastric movements in normal digestion. In man the movements of the stomach in digestion are not inhibited by acids in the stomach, that is, at least not by acids in concentrations equal to that of the gastric juice. The fact that the intensity of movements of the antrum increases as the gastric digestion advances may even indicate that a certain degree of free acidity facilitates the movements of digestion. At first it occurred to us that since acid in the stomach inhibits the hunger contractions, but not the digestion contractions, the mechanisms involved in these two types of gastric activity are different, at least as regards the character of the afferent impulses from the gastric mucosa. But on further reflection it became apparent that this is not necessarily the case, for the digestive movements involve primarily the pyloric end, while the hunger movements (as studied by our method) involve the fundus of the stomach. It is possible that acid stimulation of the nerve-endings in the gastric mucosa leads, reflexly, to a temporary inhibition of the fundus and to peristalsis of the pyloric region of the stomach.