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]

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

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]

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 (