[Illustration: The Mucous Membrane of the Ilium, highly magnified. 1, cellular structure of the epithelium, or outer layer; 2, a vein; 3, fibrous layer; 4, villi covered with epithelium; 5, a villus in section, showing its lining of epithelium, with its blood vessels and lymphatics; 6, a villus partially uncovered; 7, a villus stripped of its epithelium; 8, lymphatics or lacteals; 9, orifices of the glands opening between the villi; 10, 11, 12, glands; 13, capillaries surrounding the orifices of the gland.]
1. The Bile is secreted by the liver. This gland weighs about four pounds, and is the largest in the body. It is located on the right side, below the diaphragm. The bile is of a dark, golden color, and bitter taste. About three pounds are secreted per day. When not needed for digestion, it is stored in the gall cyst. [Footnote: A gall bladder can be obtained from a butcher, and the contents kept in a bottle for examination.] Its action on the food, though not fully understood, is necessary to life. [Footnote: The bile is produced, unlike all the other animal secretions, from venous blood; that is, the already contaminated blood of the portal vein. Its complete suppression produces symptoms of poisoning analogous to those which follow the stoppage of respiration, and the patient dies, usually in a comatose condition, at the end of ten or twelve days.—DALTON. The alkaline bile neutralizes the acid contents of the stomach as they flow into the duodenum, and thus prepares the way for the pancreatic juice. It has also a slight emulsifying power (note, p. 167).]
2. The Pancreatic Juice is a secretion of the pancreas, or "sweetbread"—a gland nearly as large as the hand, lying behind the stomach. It is alkaline, and contains a ferment called trypsin. This juice has the power of changing starch to sugar. Its main work, however, is in breaking up the globules of fat into myriads of minute particles, that mix freely with water, and remain suspended in it like butter in new milk. The whole mass now assumes a milky look, whence it is termed chyle (kile) and passes on to the small intestine. [Footnote: It is curious to observe that while the gastric juice is decidedly acid, the fluids with which the food next comes into contact are alkaline. It is thus submitted to the operation alternately of alkaline, acid, and again of alkaline secretions. In the herbivora there is also a second acid juice. The reason of these alternations is not known, but it can hardly be doubted that they serve to make the digestion of the food more perfect. And although the solvent power of the gastric juice is placed in abeyance when its acidity is neutralized by the alkaline fluids, yet it appears to be the case here, as in respect to the saliva, that effects are produced by the mixture of the various secretions which are poured together into the digestive tube, that would not result from either alone.—HINTON.]
3. The Small Intestine is an intricately folded tube, about twenty feet long, and from an inch to an inch and one half in diameter. As the chyle passes through this tortuous channel, it receives along the entire route secretions which seem to combine the action of all the previous ones—starch, fat, and albumen being equally affected.
IV. ABSORPTION is performed in two ways, by the veins, and the lacteals. (1.) The veins in the stomach [Footnote: The veins and the lacteals are separated from the food by a thin, moist membrane, through the pores of which the fluid food rapidly passes, in accordance with a beautiful law ("Popular Physics," p. 53) called the Osmose of liquids. If two liquids of different densities are separated by an animal membrane, they will mix with considerable force. There is a similar law regulating the interchange of gases through a porous partition, in obedience to which the carbonic acid of the blood, and the oxygen of the lungs, are exchanged through the thin membrane of the air cells.] immediately begin to take up the water, salt, grape sugar, and other substances that need no special preparation. The starch and the albuminous bodies are also absorbed as they are properly digested, and this process continues along the whole length of the alimentary canal. In the small intestine, there is a multitude of tiny projections (villi) from the folds of the mucous membrane, more than seven thousand to the square inch, giving it a soft, velvety look. These little rootlets, reaching out into the milky fluid, drink into their minute blood vessels the nutritious part of every sort of food. (2.)The lacteals [Footnote: From lac, milk, because of the milky look given to their contents by the chyle.] (p. 126), a set of vessels starting in the villi side by side with the veins, absorb the principal part of the fat. They convey the chyle through the lymphatics and the thoracic duct (Fig. 43) to the veins, and so within the sweep of the circulation.
The Portal Vein [Footnote: So named because it enters the liver by a sort of gateway.] carries to the liver the food absorbed by the veins of the stomach and the villi of the intestines. On the way, it is greatly modified by the action of the blood itself. In the cells of the liver, it undergoes as mysterious a process as that performed by the lymphatic glands, and is then cast into the circulation. [Footnote: In these cells, the sugar is changed into a kind of starch called glycogen. This is insoluble, and so is stored up in the liver, and even in the substance of the muscles, until it is needed by the body, when it is once more converted into soluble sugar and taken up by the circulation. The liver also changes the waste and surplus albuminous matter into bile, and into urea and uric acid—the forms in which nitrogenized waste is excreted by the kidneys.] The food, potent with force, is now buried in that river of life from which the body springs momentarily afresh.
THE COMPLEXITY of the process of digestion, as compared with the simplicity of respiration and circulation, is very marked. The mechanical operation of mastication; the lubrication of the food by mucus; the provision for the security of the respiratory organs; the grasping by the muscles of the throat; the churning movement of the stomach; the guardianship of the pylorus; the timely introduction by safe and protected channels of the saliva, the gastric juice, the bile, the pancreatic juice, and the intestinal fluids, each with its special adaptation; the curious peristaltic motion of the intestines; the twofold absorption by the veins and the lacteals; the final transformation in the lymphatics, the portal vein, and the liver,—all these present a complexity of detail, the necessity of which can be explained only when we reflect upon the variety of the substances we use for food, and the importance of its thorough preparation before it is allowed to enter the blood.
THE LENGTH OF TIME REQUIRED for digesting a full meal is from two to four hours. It varies with the kind of food, state of the system, perfection of mastication, etc. In the celebrated observations made upon Alexis St. Martin [Footnote: In 1822, Alexis St. Martin, a Canadian in the employ of the American Fur Company, was accidentally shot in the left side. Two years after, the wound was entirely healed, leaving, however, an opening about two and a half inches in circumference into the stomach. Through this the mucous membrane protruded, forming a kind of valve which prevented the discharge of food, but could be readily depressed by the finger, thus exposing the interior. For several years he was under the care of Dr. Beaumont, a skillful physician, who experimented upon him by giving various kinds of food, and watching their digestion through this opening. By means of these observations, and others performed on Katherine Kutt, a woman who had a similar aperture in the stomach, we have very important information as to the digestibility of different kinds of food.] by Dr. Beaumont, his stomach was found empty in two and a half hours after a meal of roast turkey, potatoes, and bread. Pigs' feet and boiled rice were disposed of in an hour. Fresh, sweet apples took one and a half hours; boiled milk, two hours; and unboiled, a quarter of an hour longer. In eggs, which occupied the same time, the case was reversed,—raw ones being digested sooner than cooked. Roast beef and mutton required three and three and a quarter hours respectively; veal, salt beef, and broiled chicken remained for four hours; and roast pork enjoyed the bad preeminence of needing five and a quarter hours.
VALUE OF THE DIFFERENT KINDS OF FOOD.—Beef and Mutton possess the greatest nutritive value of any of the meats. Lamb is less strengthening, but more delicate. Like the young of all animals, it should be thoroughly cooked, and at a high temperature, properly to develop its delicious flavor. Pork has much carbon. It sometimes contains a parasite called trichina, which may be transferred to the human system, producing disease and often death. The only preventive is thorough cooking. Fish is more watery than flesh, and many find it difficult of digestion. Like meat, it loses its mineral constituents and natural juices when salted, and is much less nourishing. Oysters are highly nutritious, but are more easily assimilated when raw than when cooked. Milk is a model food, as it contains albumen, starch, fat, and mineral matter. No other single substance can sustain life for so long a time. Cheese is very nourishing—one pound being equal in value to two of meat, but it is not adapted to a weak stomach. (See p. 322.) Eggs are most easily digested when the white is barely coagulated and the yolk is unchanged. Bread [Footnote: Very fresh bread, warm biscuit, etc., are condensed by mastication into a pasty mass that is not easily penetrated by the gastric juice, and hence they are not healthful. In Germany bread is not allowed to be sold at the baker's till it is twenty-four hours old—a wise provision for those who have not strength to resist temptation. This rule of eating may well be adopted by every one who cares more for his health than for a gratification of his appetite.] should be made of unbolted flour. The bran of wheat furnishes the mineral matter we need in our bones and teeth, gives the bulk so essential to the proper distension of the organs, and by its roughness gently stimulates them to action. Corn is rich in fat. It contains, however, more indigestible matter than any other grain, except oats, and is less nutritious than wheat. [Footnote: Persons unaccustomed to the use of corn find it liable to produce derangement of the digestive organs. This was made fearfully apparent in the prisons of Andersonville during the late civil war. The vegetable food of the Federal prisoners had hitherto been chiefly wheat bread and potatoes—the corn bread so extensively used at the South being quite new to most of them as a constant article of diet. It soon became not only loathsome, but productive of serious diseases. On the other hand, it was the principal article in the rations of the Confederate soldiers, to whom habit made it a nutritious and wholesome form of food, as was shown by their endurance.—FLINT, Physiology of Man, Vol. II, page 41.] The Potato is two thirds water,—the rest being mainly starch. Ripe Fruits, and those vegetables usually eaten raw, dilute the more concentrated food, and also supply the blood with acids, which are cooling in summer, and useful, perhaps, in assimilation.
THE STIMULANTS.—Coffee is about half nitrogen, and the rest fatty, saccharine, and mineral substances. It is, therefore, of much nutritive value, especially when taken with milk and sugar. Its peculiar stimulating property is due to a principle called caffeine. Its aroma is developed by browning, but destroyed by burning. No other substance so soon relieves the sense of fatigue. [Footnote: In the late civil war, the first desire of the soldiers upon halting after a wearisome march, was to make a cup of coffee. This was taken without milk, and often without sugar, yet was always welcome.] Taken in moderation, it clears the intellect, tranquilizes the nerves, and usually leaves no unpleasant reaction. It serves also as a kind of negative food, since it retards the process of waste.