The mucous membrane of the small intestine is richly supplied with blood vessels and contains glands that secrete[pg 152] a digestive fluid known as the intestinal juice. The membrane is thrown into many transverse, or circular, folds which increase its surface and also prevent materials from passing too rapidly through the intestine. One important respect in which the small intestine differs from all other portions of the food canal is that its surface is covered with great numbers of minute elevations known as the villi. The purpose of these is to aid in the absorption of the nutrients as they become dissolved (Chapter XI).
The muscular coat of the small intestine is made up of two distinct layers—the inner layer consisting of circular fibers and the outer of longitudinal fibers. These muscles keep the food materials mixed with the juices of the small intestine, but their main purpose is to force the materials undergoing digestion through this long and much-coiled tube.
The outer, or serous, coat of the small intestine, like that of the stomach, is an extension from the general lining of the abdominal cavity, or peritoneum. In fact, the intestine lies in a fold of the peritoneum, somewhat as an arm in a sling, while the peritoneum, by connecting with the back wall of the abdominal cavity, holds this great coil of digestive tubing in place (Fig. 64). The portion of the peritoneum which attaches the intestine to the wall of the abdomen is called the mesentery.
Most of the liquid acting on the food in the small intestine is supplied by two large glands, the liver and the pancreas, that connect with it by ducts.
Fig. 71—Abdominal cavity with organs of digestion in position.
The Liver is situated immediately below the diaphragm, on the right side (Figs. 71 and 72), and is the largest gland in the body. It weighs about four pounds and is separated into two main divisions, or lobes. It is complex in structure and differs from the other glands in several particulars. It receives blood from two distinct sources—the portal vein[pg 154] and the hepatic artery. The portal vein collects the blood from the stomach, intestines, and spleen, and passes it to the liver. This blood is loaded with food materials, but contains little or no oxygen. The hepatic artery, which branches from the aorta, carries to the liver blood rich in oxygen. In the liver the portal vein and the hepatic artery divide and subdivide, and finally empty their blood into a single system of capillaries surrounding the liver cells. These capillaries in turn empty into a single system of veins which, uniting to form the hepatic veins (two or three in number), pass the blood into the inferior vena cava (Fig. 72).
Fig. 72—Relations of the liver. Diagram showing the connection of the liver with the large blood vessels and the food canal.
The liver secretes daily from one to two pounds of a liquid called bile. A reservoir for the bile is provided by a small, membranous sack, called the gall bladder, located on the underside of the liver. The bile passes from the gall bladder, and from the right and left lobes of the liver, by three separate ducts. These unite to form a common tube which, uniting with the duct from the pancreas, empties into the duodenum. Though usually described as a digestive gland, the liver has other functions of equal or greater importance (Chapter XIII).