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.
IV.
Of the absorption of the materials thus prepared it is not necessary to say much in a work of this compass, but the absorption of oxygen is too important to be passed over.
Oxygen is required by the body pure, and, as it is uncombined with anything in the air, it needs no digestion to free it. A special organ, however, is necessary to absorb it. This is the lung. The lungs originate, just like a gland, by a pouching of the alimentary canal near its origin, but differ from a gland in their cells being very much flattened, to offer a large surface to the air on one side and to the bloodvessels on the other. Incessantly during life air is being drawn into the lungs; that the cells to which it is there exposed may transfer its oxygen to the blood; and then, after the cells have also transferred the carbonic acid gas from the blood to the air, driven out again to be replaced by fresh.
(The mechanical means by which the lungs are filled and emptied come under another heading.)
V.
Food having been absorbed by cells set apart for the purpose, the next problem is, How is it distributed to those specialized for other work? The medium for this distribution is a liquid called lymph. All the spaces in the body are filled with lymph, all the organs bathed with it, every cell moistened with it; yet it is comparatively stagnant, and the food has to be conveyed from the walls of the alimentary canal to the lymph in the neighbourhood of the cell requiring nourishment by a more expeditious agent. This is done by the blood.