III.
In man, the digestive process may be divided into three stages. They are arranged progressively, so that each clears the way for the next, and take place in the mouth, the stomach, and the upper part of the small intestine, the rest of the canal being mainly occupied in absorption.
Diagram 10.—General Scheme of the Alimentary Canal, with its Offshoots—Lungs and Glands.
By far the largest proportion of the food is carbohydrate, in some form, so one naturally expects the first stage of digestion will deal with the constituents which represent this class. This is the case. The food is taken into the mouth in small quantities and ground up with the teeth, during which process it is subjected to the action of the saliva. This fluid, which is the secretion of three pairs of glands, converts a large proportion of the carbohydrates, starch, cane-sugar, etc., into a very simple sugar which is absorbed directly it reaches the stomach.
One of the most sensational discoveries of the physiologist has been that the saliva leaving the gland does not contain the ferment necessary to effect this change until it has been subjected to the action of putrefactive bacteria. These, fortunately for us, it is pleasant to know, simply swarm in the mouth.
When the food is swallowed, it passes very rapidly down the first part of the alimentary canal, which is straight, and is then kept for some time in the stomach. The stomach differs from the rest of the canal in several particulars, among them the following: it is a large cavity, and is closed at each end by a valve to keep the food in until it has been thoroughly treated, and it deals with the whole mass of food taken at a meal at one time, and yet has no contrivances for increasing its surface.
Here the food is subjected to a most important and searching examination. Enclosed in this bag, it is thoroughly mixed with weak hydrochloric acid, secreted by numerous glands, and kept churning round and round by the muscular action of its walls, that the contents may be kept well mixed. The acid is just strong enough to kill protoplasm, and hence the putrefactive bacteria which were necessary in the mouth, but would be a very doubtful blessing in the interior of the body, are disposed of. Other things are also killed. Not only does the stomach execute intruding bacteria, but it also kills a good deal of our food. Fruit and salad consist largely of still living cells, and occasionally there is bigger game, e.g., oysters. One thing, however, the acid does not kill, and that is the cells lining the stomach, and it may as well be said here that the parts of the body exposed to ferments have the very necessary power of resisting them, so that a normal animal does not digest itself.
The stomach, however, is a kitchen as well as a slaughter-house. The gastric juice, or secretion of all the glands opening into it, contains, besides the acid, two important ferments, both of which act on proteids. Carbohydrates are absorbed, but not digested, in the stomach, as acid destroys saliva. One of the ferments is rennet, an article familiar to the culinary profession, which solidifies milk. The other acts on proteids generally, converting them ultimately into a very simple form, peptone, which is absorbed at once. How much of the proteid in the stomach is converted into peptone is not known, for the action of acid alone is sufficient to enable it to be absorbed. A solution of proteid, e.g., white of egg, is quite altered if made slightly acid; it no longer coagulates when boiled, but the change of the most practical interest is that, if injected into the veins, it seems to become part of the blood, while ordinary proteids act as poisons.
The peptonizing ferment, however, has one very important function: it digests the collagen of the connective tissue, the substance which becomes gelatin when boiled. The reason why this is so important is not only that nothing else in the body affects it, but that fat is enclosed in it, and if it were not thus set free would pass through the body unabsorbed.
The final stage is the digestion by the pancreatic juice. After the food has been exposed for some time to the gastric juice, it is allowed to escape a little at a time from the stomach, and continues its way along the alimentary tube. Before it has gone many inches it comes to the openings of two ducts, those of the liver and the pancreas, and immediately the acid stimulates them, and the glands pour out their secretion. That of the liver is largely excretion or refuse from the blood without direct action on the food, but it enables the pancreatic juice to do its work by making the food again alkaline, and stimulates the muscular coats of the intestine to force its contents along. That of the pancreas is the most important digestive fluid in the body, containing many ferments; it acts alike on proteids, carbohydrates, and fats—in fact, digests everything—so that the rest of the long tube is freed from any more laborious duty than absorbing them as they pass.
Note.—The digestive ferments are now prepared for examination by chopping up the gland and placing it in glycerine; this extracts the ferment and preserves it from the action of bacteria. The first experiments on digestion, however, constitute one of the romances of physiology. A Canadian named St. Martin got into trouble with Red Indians whilst in the United States, America, and was shot through the body. The surgeon who attended him was unable to make the wound close, and when it healed there remained an opening in the man’s body communicating directly with his stomach. The surgeon, Beaumont, saw possibilities in this, and, obtaining gastric juice from his patient, made those classical experiments which entitled him to a place among the fathers of physiology. Americans do well to be proud of Beaumont, for it cost him many sacrifices, and his patience and courage are above praise. Not only was he devoid of all but the crudest appliances out in the backwoods, but his subject proved intractable and mercenary. No sooner did he discover his value than he crossed the border, and refused to return except upon exorbitant payment. Even after this had been arranged, he repeated the performance whenever he thought fresh extortion possible. In spite of these difficulties, the investigations proved wonderfully accurate and complete.