The action of carbon dioxide in the cavity of the empty stomach was studied in two ways: (i) by introduction of water charged with CO,, (2) by introduction of COa gas. An excess of carbon dioxide in the blood of the abdominal vessels is supposed to augment the tonus of the digestive tract, although some recent work of Hooker throws doubt on this interpretation of the stimulating action on intestinal peristalsis by partial asphyxia. An excess of COa is sometimes found in the gaseous contents of the empty or partly filled stomach. It is known, furthermore, that carbon dioxide in sufficient concentration acts as a powerful stimulus to the nerve-endings in such membranes as those of the mouth and nose and of the cornea and conjunctiva.
Carbon dioxide in the cavity of the empty stomach was at first considered a possible stimulus to the gastric hunger contractions, but this hypothesis proved entirely erroneous. So far as the carbon dioxide in the cavity of the stomach affects the hunger movements, the influence is in the direction of inhibition.
Water saturated with CO2 under pressure has practically no more effect than similar quantities of pure water. It produces the same degree of temporary inhibition without any after-effect of augmentation. As such carbonated water stimulates the nerve-endings in the mouth in the characteristic way, it follows that the nerve-endings in the stomach are less affected by CO2 than are the nerve-endings in the mouth.
When the CO* is forced into the stomach in the form of gas and under pressure, the results are complicated by the mechanical action of the gas in forcibly distending the walls of the stomach and raising the intragastric pressure, and hence increasing the pressure on the balloon in the fundus. A sudden and forcible distension of the empty stomach, no matter how produced, leads to a few strong contractions. This factor can be fairly well controlled by introducing the gas slowly. When this precaution is taken, the empty stomach can be considerably distended with CO2 gas, without any marked effect, either on the tonus or on the hunger contractions. But the chemical effect of CO2, so far as it is demonstrable at all, is in the direction of inhibition.
It will undoubtedly occur to the reader that this slight inhibition by the CO2 may be an instance of "psychic" inhibition from the distress of an overdistended stomach. This possibility has been guarded against. In the first place, the stomach was not distended to the point of painfulness by the carbon dioxide. Furthermore, the stomach cavity was irrigated, so to speak, with the gas without raising the intragastric pressure perceptibly, by introducing the inlet tube to the cardiac end and allowing the gas to escape by way of the mouth, or, in the case of Mr. V., by the open fistula. Under these conditions the same slight inhibitory effects were recorded without signs of primary or secondary augmentation. It is thus clear that so far as carbon dioxide in the gastric cavity affects the gastric tonus and hunger contractions at all the action is in the direction of inhibition. This is probably due to the acid stimulation of the nerve-endings in the mucosa.
The introduction of air into the empty stomach has no effect whatever on the tonus and the hunger contractions, provided the stomach is not overdistended by the air, or the air introduced rapidly and under such pressure as to cause sudden and forcible distension of the stomach walls. This leads to a few contractions. But the same thing is produced by sudden inflation of the balloon in the fundus. It is therefore purely mechanical. Oxygen in greater concentrations than that of the air has not been tried. But it is evident that the 20 per cent oxygen of the air acts neither favorably nor unfavorably on the hunger movements.
The fact that nothing but inhibition is produced by substance acting on the gastric mucosa suggests that this may be in every case a "psychic" inhibition masking any weak action that may be of a positive or augmentation type. The very consciousness that these substances were introduced into the stomach for experimental purposes might be the primary element in this possible psychic inhibition. That cerebral states may inhibit the gastric hunger movements is certain from results both on man and on dogs. In one instance, when preparing to introduce 200 c.c. of 0.5 per cent acidic acid into the stomach in the midst of the period of powerful hunger contractions, Mr. V. somehow thought that we intended to introduce that much concentrated acid (or vinegar). As we were going about with the preparations it was noticed that the stomach contractions suddenly became very feeble. Mr. V. looked worried. We inquired if he did not feel right, and he asked if we intended to put all that vinegar into the stomach. " It will surely hurt me," he said. To assure him, the author drank half of the acid himself, and then asked him to take a mouthful of it. Then he laughed and said, "Oh, I thought it was pure vinegar." In two minutes after the mental stress and anxiety was over the hunger contractions returned to their normal rate and amplitude.
The following facts speak against the possibility of the results being due to psychic inhibition: (1) There was no evidence that Mr. V. or any of the subjects were in any way afraid, displeased, disgusted, or impatient with the experiments. (2) The direct proportion between the quantity and the concentration of the substance introduced into the stomach and the degree of inhibition produced is contrary to the hypothesis of a psychic inhibition. The displeasure or disgust ought to have been practically the same on introduction of 0.1 per cent and of 0.5 per cent HCl, of 1 per cent and 10 per cent alcohol, as in most cases the subjects did not know the strength of the material used. (3) In many cases the subject "was purposely deceived as to the nature of the material, using water for acids and vice versa. The stomach reaction was invariably in accordance with the substance actually introduced.
We feel satisfied, even on the basis of the tests on man, that psychic inhibition plays no rdle in these results. But to meet the possibility once and for all, we have repeated and confirmed all of the above tests on dogs. The parallel on the two series on man and dog is complete. May not psychic inhibition play a rdle in the tests on dogs ? It does not, and for the following reasons: (1) The dogs could not have known either the difference between the substances introduced into the stomach or the different concentrations of the same substance. (2) Tests were made during sleep and without the animal waking up. The results were the same. (3) Psychic inhibition of the gastric hunger movements in dogs is invariably of much shorter duration than the inhibition caused by acids, alkalies, and alcoholic beverages.
It is therefore clear that these results on man are fundamental facts in the physiology of the stomach and not primarily dependent on afferent impulses that enter consciousness.