Digestion in the soon as the food reaches the stomach that organ becomes active. By the contractions of its muscular walls the food is moved about, and mixed with gastric juice poured out of the gastric glands as described on page 198. The action of the juice is aided by the heat of the parts. The gastric juice is acid owing to the presence of a small quantity of acid, usually hydrochloric acid. It contains, besides, a ferment called pepsin. It is owing to the presence of the acid and pepsin in the stomach that digestion is performed. The action of the juice is on albuminous substances. Now albuminous sub stances are not soluble in water, nor can they pass through animal membranes, but by the action of the juice they become converted into what are called which are both soluble and capable of passing through mem brfines. The juice of the stomach has an action on fat to this small extent, that fat consists of a drop of oil in an albuminous sac, and by the juice this sac becomes dissolved, so that the oil is freed, but no further action on it is effected. Thus it is only on albuminous food-stuffs that gastric juice acts. As a result of the action in the stomach the food becomes converted into a semi-fluid mass called chyme.
Digestion similar to that performed in the stomach can be artificially produced. An acid solution of pepsin is required. This is obtained in the following way. The stomach of a pig is taken, opened up, and very gently washed with a stream of water. The inner coat—the mucous membrane—is then stripped off, cut into very small pieces, placed in a bottle among glycerine and water containing a small amount of hydrochloric acid (li drachm to every 100 ounces). It is allowed to stand for several (lays. The glycerine-and-acid solution extract the pepsin from the glands of the stomach. If now some small pieces of boiled meat, boiled egg, fish, &c., be put into a glass vessel with some water, and if a small quantity of the glycerine extract of pepsin be added, and the whole kept at the temperature of the body— about 100° Fahr.,—in a few hours the meat will have undergone digestion. Pepsin wine may be made by adding sherry wine to the gly cerine extract. The pepsin powder that may be obtained from chemists is prepared from the pig's or calf's stomach. Besides pepsin, another ferment is contained in the gastric juice, called rennin, which produces a curdling action upon milk. Rennet, which is used for curdling milk, is a preparation of the calf's stomach, and owes its property to the rennin it contains. Indeed, what happens to milk on the addition of rennet is precisely similar to what happens to milk passed into the stomach. Owing to the acid of the stomach and rennin, aided by the heat of the parts, the milk is curdled, and separates into curd and whey. The curd contains the main albuminous con stituent of the milk, casein, which the pepsin forthwith proceeds to attack. It is thus seen that the curdling of milk in the stomach is a first and essential part of the process of digestion. This it is desirable to note, because many mothers, after permitting a child to drink too much milk, are alarmed to see the child, after some time, vomit curdled milk. The vomiting does not alarm them, but the curd does. The explanation usually is that the stomach, being overloaded, rejects some of the milk in a curdled condition, because it has already conic under the influence of the gastric juice.
Conditions of digestion in the stomach.
The acid is as necessary to the process as the pepsin, for it has been shown in artificial digestion that pepsin alone cannot act upon albuminous food in a proper way. Hence if
the gastric juice be too feebly acid, or if the acidity be destroyed by soda or other alkalies, for instance, digestion will be imperfect. But an excess of acid equally interferes with the process; thus what is called "acidity of the stomach" produces indigestion in this way. Excess of alcohol also impedes the process.
Overfulness of the stomach will impede the movement of the walls, and therefore the mix ture with the juice, and so hinder digestion.
It has been seen (p. 198) how a due supply of blood is necessary for the formation of the digestive fluid. If the blood be occupied elsewhere, as it may be if active exercise be engaged in immediately after food, there may be insufficient for digestive purposes, and a delay in the process results. The secretion of the juice is also undoubtedly under the control of the nervous system. For this reason, no doubt, active brain work immediately after food, either by diverting the nervous activity or by diverting the blood supply, the brain by its activity making great demands on both, may produce indigestion. Gentle exercise, therefore, and repose of mind, are conditions favourable for digestion.
It may be that owing to ill health the blood supply may be poor, and the nervous tone indifferent, so that indigestion may be only one symptom of general ill health.
The stomach, like all other organs of the body, should have periods of rest following its periods of activity, and these periods should follow one another regularly.
Time required for kinds of food require varying times for diges tion. This was proved by a remarkable set of experiments performed, in 1838, on a man named Alexis St. Martin, by Dr. Beaumont. St. Martin had an opening made in the front wall of the stomach by a gunshot wound. Even after complete healing of the wound a small opening was left through which the mucous membrane of the stomach could be seen, and through which substances could be introduced into the stomach or withdrawn from it. It was found that rice and tripe were digested most speedily, the time required being one hour. Eggs, salmon, trout, apples, and venison occupied an hour and a half; tapioca, barley, milk, liver, and fish, two hours: turkey, lamb, and pork, two hours and a half; beef, mutton, and fowls, about three and a half hours, and veal even longer. (Further details, p. 106, Vol. II.) Absorption by the have seen that the mucous membrane of the stomach is richly supplied with blood-vessels. The blood flowing in them is separated from the semi-liquid food only by a thin animal parti tion. There is, therefore, no impediment to an interchange taking place between the blood and the food. What the nature of that change will be we have already learned (p. 194). Water, along with any substances in the food that have becotne dissolved, will pass through the partition and gain access to the current of blood. Thus a considerable quantity of salts in solution, of starch that has become con verted into sugar, of albumin converted into peptone, will, without further delay, gain en trance to the blood. Starch that has escaped the action of saliva, albumin that has escaped the action of gastric juice, and fats, will remain in the food, and will be passed on into the small intestine, where the digestive process is continued by other juices.