The products of ferment activity have a retarding effect upon the reaction; it is interesting to note that only those products exert this inhibition which have a definite stereochemical structure and it is supposed that this may be similar to that of the ferment. Thus the activity of the lactase which hydrolyzes lactose to glucose and galactose is hindered by galactose but not by glucose.
It is possible that the combination between a ferment and its substrate is similar to that occurring between a toxin and an anti-toxin or a lysin and anti-lysin. According to the theory of Ehrlich a toxin is a molecule with a side chain, called the haptophore group, which can hook on to a tissue molecule, and another side chain, the toxophore group, to which the poisonous action of the toxin is due. The anti-toxin is supposed to neutralize the toxin by combining with the haptophore group which is therefore prevented from attaching itself to the tissues. Evidence has been brought forward to show that the combination between a toxin and an anti-toxin is not simply a chemical one. For instance, a certain quantity of toxin may be neutralized by a definite quantity of anti-toxin, but if the solution be diluted the neutralization is no longer complete. This and other considerations suggest that the combination may belong to a class of reactions partly physical in nature, known as the phenomena of adsorption, which have been observed in substances in the colloid state, that is to say, consisting of aggregates of molecules with a very large amount of surface. Near the surface of such suspended aggregates the concentration of molecules dissolved in the solution is different to that elsewhere and this affects the velocity of reactions taking place between them. Now the food-stuffs upon which the ferments act are colloid in nature, and it is probable that the ferments are too, and if this be so, it may be found that the mode of operation of the ferments of the body belongs to this physico-chemical group of reactions.
We may now follow the food-stuffs through the stomach and intestines. The food material passed from the gullet into the stomach lies in the cardiac part or fundus. In this mass salivary digestion goes on for a considerable time. Hensay found that in healthy young people 60-80 per cent of the carbohydrate food taken was dissolved in the stomach in one-half to three-quarters of an hour. It has also been shown that in rabbits, guinea-pigs, and cats salivary digestion goes on for a considerable time in the cardiac part of the stomach. In horses, pigs, and rats, the same is true, but in these animals the cardiac glands do not secrete acid. The action of ptyalin is stopped by the hydrochloric acid of the gastric juice, and the juice begins to be secreted even before food is taken into the stomach. It does not, however, permeate the mass of food lying at the cardiac end for half an hour or more, only attacking the outer portions which, as they become semi-fluid, are squeezed on into the pyloric part of the organ.
The gastric juice is a clear fluid containing water, salts, ferments, and .2-.4 per cent of hydrochloric acid. Probably the acid is combined loosely with protein, for it does not behave to reagents as a pure solution of the same strength. The presence of acid is necessary to enable the ferment pepsin to hydrolyze protein to acid albumin, albumoses and peptone; and the combination of hydrochloric acid with protein is still able to exert this favourable influence upon peptic action.
The secretion of gastric juice takes place in two phases. In the first place the thought of food, the introduction of food into the mouth, and the chewing of food all call forth a flow of juice in a hungry individual, known as the appetite juice. In a dog with an oesophageal fistula, through which the food swallowed drops out, large quantities of juice may be collected from a fistula in the stomach when the animal is allowed to chew meat. Thorough mastication has also been shown to increase the amount of juice in man. The great importance of appetite has always been recognized and is well illustrated by Pawlow's observation that if a dog has been fasting, the sight of any food will produce gastric juice, whereas if it has been recently fed, and its appetite abated, there is only a flow if food be shown to which it is specially partial. The observations made many years ago by Beaumont upon Alexis St. Martin anticipated much of the work which has since been done. More recently, experiments on man have confirmed many of the results obtained by Pawlow on dogs. If food be introduced into the stomach of a dog without its knowledge, no juice flows for a considerable time. For example, a piece of meat was placed in the stomach of a dog through a gastric fistula, and 6 grammes of it were dissolved in an hour and a half. When, however, the appetite was excited by sham feeding, the food given passing out through an oesophageal fistula, 30 grammes, or six times as much, of the meat placed in the stomach was dissolved in an equal time. Schule has found that patients to whom food was given without their knowledge through a sound, after washing out the stomach, digested but little of it. By a similar procedure Lang found that some secretion was produced by meat, but none by carbohydrate. The flow of appetite juice is excited reflexly through the vagus nerve; it is important to note that any pain or discomfort inhibits the secretion.
The second phase of the secretion of gastric juice is called forth in the stomach itself (Edkins), and is probably independent of all nervous connexions. The products of digestion apparently cause the formation of some substance in the mucous membrane which is carried by the blood to the glands and stimulates them to secrete. This method of exciting secretion has been called a chemical reflex, and the substance producing it is known as a hormone (Bayliss and Starling), from arouse or excite. The bodies which most easily cause this second flow of juice are the products of protein digestion, and it follows that the appetite juice is necessary to start digestion in order that they may be formed. Similar bodies are, however, contained in meat broth, and this has been shown to stimulate the second flow of juice in man and in dogs. When the appetite juice is wanting, broth or soup may, therefore, act in some degree as a substitute.