The same coats continue in the small intestine as were found in the stomach, but they are here much thinner and the inner coat is shaggy, like velvet, with innumerable minute processes called villi, which greatly increase the absorbing surface. In fact, the great length of the intestine as well as the presence of the villi is aimed to provide a large surface to absorb the food as it passes, an even greater increase of surface being provided by the fact that the intestinal wall is thrown into folds, the valvulæ conniventes. Each villus is covered with a layer of columnar epithelial cells and has within connective tissue, in which are found a fine capillary network and open lymph spaces from which leads a single lacteal vessel.
Closely connected with the lymphatic vessels are the solitary glands, small round bodies the size of a small pin’s head. Peyer’s glands or patches are patches of solitary glands opposite the mesenteric attachment and are largest and most numerous in the ileum. In typhoid fever they are involved and may become the seat of ulcers. There are also the glands of Lieberkühn which secrete the succus entericus.
The arteries of the small intestine, which include the superior mesenteric, are from the celiac axis and the nerves are from the superior mesenteric plexus of the sympathetic. The veins empty chiefly into the portal system.
The movements of the intestine, like those of the esophagus, are peristaltic, but the action is complicated by the fact that the tube is not straight but in coils.
Intestinal Digestion.—The food, which enters the [duodenum] as chyme, there comes in contact with the bile and the pancreatic juice, which together but unmixed enter the duodenum from their respective ducts by a common orifice. As in the stomach, the digestive juices are called forth by the presence of food. The bile is secreted in the liver, from which it flows away through the hepatic duct, which joins the cystic duct from the gall-bladder to form the common bile duct. Through this it flows into the intestine during digestion, but between whiles it passes up into the gall-bladder, where it is stored for future use and whence it is expelled when needed. When pure it is a thick, viscid liquid, varying from a bright red to a greenish-yellow in color according to the pigments present, and of an alkaline reaction. It consists chiefly of the bile pigments, biliverdin, which gives the green color, and bilirubin, which gives the red color, and of bile salts in solution, cholesterin, which probably forms the basis of many gall stones, is also present. Bile is a disinfectant to the bowel and a lubricant for the feces. How much digestive action it has is a question, but it affords the necessary alkaline medium for the pancreatic juice to act in.
The pancreatic juice is secreted by the [pancreas], from which it enters the intestine through the pancreatic duct, and is probably the most important fluid in the digestive process. It is clear, practically colorless, slightly viscid or gelatinous, and quite strongly alkaline in reaction, owing to the presence of sodium carbonate. It contains three ferments, amylopsin for the digestion of starch, trypsin for the digestion of proteins, and steapsin for the digestion of fats. By it, as by the saliva, starch is turned into sugar or maltose, in which form it is absorbed, while proteins are converted into peptones, as they are in the stomach. Since, however, fats are acted on nowhere else, the chief function of the pancreatic juice may be considered to be the digestion of fats. Having broken through their albuminous envelope, it divides them into glycerine and fatty acids and then emulsifies them with the assistance of the bile.
The food also comes in contact with the succus entericus, a juice secreted by the glands of Lieberkühn in the small intestine, whose chief action is the conversion of sugar into glucose.
Absorption.—As the food is absorbed from the intestine it is liquid and entirely digested and is known as chyle. Practically all absorption takes place from the small intestine, though there is a little in the large intestine. It takes place in two ways: 1. through the portal vessels and 2. through the lacteals, which are the lymphatic vessels of the small intestine. Fats are absorbed practically entirely by the lacteals. They enter the cells covering the villi, travel thence to the lymph spaces, and so into the lacteal or main lymph channel, whence they are carried to the thoracic duct and the general circulation. From the blood they are absorbed as fat and stored up as adipose or fatty tissue, which is found throughout the body in connective tissue about the organs. Organic salts and water are for the most part absorbed by the portal system, which they reach through the [capillaries] of the villi and through which they go to the [liver]. Starches, in the form of sugar, pass between the cells of the villi into the lymph spaces, from which they are taken up by the capillaries. On the way to the liver maltose becomes dextrose. Proteins, in the form of peptones, pass through the layer of epithelial cells to the lymph spaces and then to the capillaries, an active part being taken by the cells. By the time they reach the liver the peptones have been changed back into proteins. In fact, peptones seem to have some poisonous effect upon the blood if they get into it as such.
The Large Intestine.—The large intestine differs from the small in size and in fixity of position, lying curved in horseshoe shape above and around the small intestine. It is five or six feet long, large in caliber, and is thrown into crosswise folds. It has the same four coats as the small intestine, but the mucous coat is pale and smooth, without villi. Its glands are the crypts of Lieberkühn and the solitary glands. The arteries are branches of the superior and inferior mesenteric and the nerves come from sympathetic plexuses.
The blind sac lying in the right iliac fossa, with which the large intestine begins, is called the [cecum], and into this the ileum opens, the ileo-cecal valve preventing regurgitation. Just below the ileo-cecal opening is the vermiform appendix, a narrow, worm-like tube with a blind end, varying in length from one to nine inches, but generally about four and one-half inches long, which, so far as is known, is functionless as well as dangerous. People have been born without an appendix and it has in rare instances grown again after operation. Its base is located in the living by McBurney’s point, a point two inches from the anterior superior spine of the ilium on a line drawn from the spine to the umbilicus.