The work on man led to the conclusion that any substance capable of stimulating the nerve-endings in the gastric mucosa causes inhibition of the tonus and hunger contractions, and inhibition only, as there is no evidence of any increase in the gastric tonus or hunger contractions following the primary inhibition. The experiments on dogs were undertaken primarily to determine the character of this reflex, that is, whether central, or local, or both. The liquids were introduced into the stomach through the fistula by means of a soft rubber tube, so that swallowing acts and the stimulation of nerve-endings in the mouth, the pharynx, and the esophagus were completely eliminated.
The observations were made on six dogs with all the extrinsic gastric nerves intact; on six dogs with the splanchnic nerves cut; and on four dogs with complete section of both of the vagi and the splanchnic nerves. The results on the normal dogs are practically identical with those on man. Gastric juice (human; canine), weak acids and alkalies, brandy, wines, and beer introduced directly into the empty stomach during hunger contractions produce immediate inhibition of the gastric tonus and contractions. Thus the same quantity of gastric juice or wine seems to cause more prolonged inhibition in dogs showing the type I than in the dogs showing type III hunger rhythms. The duration of these inhibitions can best be studied in the dogs showing types II and III of hunger contractions, as these two forms are practically continuous, so that the errors from spontaneous periods of relative quiescence are eliminated. In normal dogs showing contractions of types II and III, 25 c.c. gastric juice or 0.5 per cent HCl usually causes complete inhibition for 20 to 30 minutes. The return of the hunger contractions is always gradual. In like manner, 25 cc of beer will inhibit for 15 to 25 minutes. In one case 50 c.c. of beer caused complete inhibition for one hour.
If these substances are introduced into the stomach of dogs during a period of relative quiescence and tonus relaxation the only effect appears to be a still greater tonus relaxation and prolongation of the quiescent period. In some cases one or two hunger contractions follow immediately on introducing the material into the stomach. We are inclined to attribute these contractions to the mechanical distension of the stomach wall rather than to stimulation of nerve-endings in the mucosa. This phenomenon was never observed when the stomach was in strong tonus and hunger contractions.
The influence on the hunger contractions of CO, in the stomach cavity is the same in dog and man. The experiments on dogs were made with water saturated with CO2 and with CO, gas. When the gas was employed, at times enough of it was passed into the stomach via the fistula to cause escape of the gas through the esophagus. The water saturated with CO, has practically the same action as ordinary water, that is, a slight temporary inhibition without any after-effect of the nature of increased tonus or contractions. This is true whether the carbonated water is introduced during active hunger contractions or during relative quiescence. The CO, gas usually initiates some contractions if introduced into the stomach during a period of quiescence. This is evidently due to mechanical distension of the stomach walls and not to chemical stimulation of nerve-endings in the mucosa. If the empty stomach is in vigorous tonus and hunger contractions the CO, gas causes a slight temporary inhibition without any stimulating after-effect. This temporary inhibition is in all probability due to a weak acid stimulation in the nerve-endings in the mucosa.
The inhibition of the gastric tonus and hunger contractions by acids, alkalies, alcohol, etc., in the stomach cavity persists after section of the splanchnic nerves, but it is on the whole less complete and of shorter duration than in dogs with all the extrinsic gastric nerves intact. This applies to all substances used in this series of experiments. When, as in the present series, the test with each substance is repeated at least ten times on each animal, some variation in the intensity and duration of the inhibition appears. That is to be expected, because the degree of inhibition depends on several variable factors, such as the excitability of the nerve-endings in the mucosa, the excitability of the Auerbach plexus and of the central nervous system, the tonus of the stomach, etc. It is therefore true that the most pronounced inhibition observed after section of the splanchnic nerves may be as marked as the feeblest inhibition obtained in the normal dogs. But when all the results in the two series of dogs are compared there is no question but that section of both splanchnic nerves diminishes the inhibition following chemical stimulation of the gastric mucosa by acids, alkalies, alcohol, etc.
Several explanations of this fact suggest themselves. (1) Since section of the splanchnic nerves in dogs increases on the whole the tonus and the hunger contractions of the empty stomach, the diminished inhibition may be due to this greater vigor of the stomach rather than cutting the efferent path of a long reflex. We do not think that this is the main or important factor, because the typical marked inhibition is obtained in normal dogs, even when the stomach shows as vigorous tonus and hunger contractions as the maximum shown by dogs with the splanchnic nerves severed. Moreover, the inhibition is still incomplete in splanchnetomized dogs that show relatively feeble hunger contractions. (2) The substances stimulate afferent vagi nerve-endings in the mucosa, and the afferent vagi impulses via conscious or subconscious centers finally stimulate the efferent inhibitory neurones in the splanchnic system. It is well known that the vagi carry afferent fibers \ from the stomach mucosa and that the splanchnic nerves carry inhibitory fibers to the stomach. The present experiments give the first intimation that the afferent vagus and the efferent splanchnic systems are so intimately associated in gastric motor reflexes. It is possible that the reflex also involves the adrenal glands, so that the inhibition mentioned above is to be accounted for, in part, by the depressor action of an increased output of epinephrin.