The total amount of gastric juice produced is proportional to the amount of food, but the rate of flow varies with the nature of the food ; a dry food, such as bread, exciting more flow in the first hour, whereas with meat and with milk the maximum is reached later.
The mass of food lying in the fundus of the stomach is gradually dissolved by the gastric juice. The muscular wall of the cardiac end is at this time in a state of tonic contraction ; as the solid material becomes fluid it is squeezed by this pressure into the pyloric end. The division between the cardiac and pyloric parts is known as the pyloric antrum, and is easily distinguishable during life, though not in dissecting room specimens ; it may often be seen in the post-mortem room ; indeed, the pyloric part is sometimes so contracted as to be similar in appearance to the duodenum. About half an hour after the food has passed into the stomach waves of constriction begin at the antrum and pass towards the pylorus. Each wave takes about half a minute in its passage, and as a wave starts about every ten seconds two or three can be seen in the pyloric part of the stomach at once. These energetic contractions of the pyloric end can exert a considerable pressure and have the effect of moving about and rubbing down the food. When the hydrochloric acid of the gastric juice has combined with all the protein present the further secretion of juice results in the presence of free acid, and this acid appears to stimulate the muscular walls to yet more active contractions.
These movements of the stomach have been observed in the exposed organ, in normal animals and man, and by means of the X-rays, food mixed with nitrate or carbonate of bismuth having been previously given. They have also been seen in the excised organ and are therefore independent of its nervous connexions. Nevertheless, the muscular walls of the stomach are influenced by nerves, for stimulation of the vagus has been shown to increase the movements; and in animals in which these nerves have been divided the passage of food out of the stomach is much delayed. As the stomach contracts the body of the organ is lifted up towards the fixed cardiac orifice, so that the pylorus is the lowest point, and when it opens the chyme will pass out of it by gravity. Link found in a number of observations on man that the food was discharged more rapidly if the patient lay on the right side.
The rate at which food leaves the stomach varies according to its nature. Cannon found with the X-rays that carbo-hydrate foods, such as potatoes, begin to pass through the pyloric orifice without much delay. Proteins, except egg white, do not leave at all in the first half hour, and pass out more slowly. Fats begin to leave before proteins, but take a still longer time to be completely discharged. The explanation of these facts is of considerable interest. It appears that the pylorus begins to open as soon as a certain proportion of free acid is present. Carbo-hydrate food has no neutralizing effect upon the gastric juice and consequently the degree of acidity required to bring about relaxation of the pylorus is soon attained. Proteins, on the contrary, combine with hydrochloric acid and the acid first poured out must satisfy the affinity of the protein before any can be free in the stomach and stimulate the pyloric mucous membrane. It must be remembered that the proteolytic activity of pepsin can go on without free acid provided that the combination of acid with protein be present. If this explanation be the true one we should expect that the discharge of food from the stomach would be delayed by the addition of alkali and hastened by acid, and this was found by Cannon to be the case.
When a sufficient degree of acidity has been reached the pylorus opens and allows a small portion of the chyme to pass into the duodenum. The presence of acid in the duodenum causes a closure of the pylorus (Hirsch and Serdjukow) so that each portion of acid chyme which is squirted into the duodenum will, until it has been neutralized by the alkali of the bile and pancreatic juice, inhibit the passage of any more. The interval between the discharges is longer with protein than with carbohydrate, because not only has the free acid to be neutralized but also that in combination. As soon as the chyme is rendered neutral the inhibition is suspended, the pylorus again opens in response to the stimulus still acting in the stomach and a new consignment of chyme is forwarded. While this is going on the food remaining in the cardiac end of the stomach is being gradually dissolved in its turn and passed on to be churned in the pyloric end. The amount of acid juice does not increase indefinitely, for hydrochloric acid in sufficient quantity inhibits the secretion of more.
Fat leaves the stomach very slowly; its presence in the duodenum has, like acid, the effect of causing the pylorus to close. Consequently fat does not accumulate in the small intestine, but is dealt with by the bile and the pancreatic juice in small quantities at a time.
Water does not appear to be absorbed in the stomach but is passed on quickly into the duodenum.
In the small intestine the chyme is mixed with the pancreatic juice, the bile, and the intestinal juice, which are poured out into the lumen of the gut just at the time that the material upon which they are to act arrives. This is brought about by a most interesting mechanism for the discovery of which we are indebted to Bayliss and Starling. The acid of the chyme as it passes into the duodenum acts upon a substance in the mucous membrane known as prosecretin, with the result that a body, named secretin, is formed and carried by the blood vessels to the pancreas which is thereby stimulated to produce its digestive juice. At the same time, and by the same means, a flow of bile and of the intestinal juice of the upper part of the small intestine is also excited. All these secretions are therefore stimulated by the hormone secretin, the mechanism being chemical and not nervous; for in the case of the pancreas it has been shown that the secretion is independent of any-nervous connexions ; all that is necessary is that acid shall be introduced into the duodenum and that the blood vessels shall be intact.