The Intestines are divided into three parts- the duodenum; the small intestine, consisting of jejunum and ileum; and the large intestine. The structure of the duodenum and small intestine is alike; they consist of two muscular coats- an inner circular layer and an outer longitudinal layer. The mucous membrane is covered by a number of villi, small processes about 1/40 inch long, and closely set all over the surface; they are for the purposes of absorption. In order to increase the surface, the mucous membrane is arranged in a series of circular folds called valvulae conniventes. On the surface of the mucous membrane are found also Peyer's patches, which are small aggregations of lymphoid tissue about 1/2 inch long; there are about thirty-five to forty of them usually found, most marked in young subjects.

Duodenum

The Duodenum is really the first part of the small intestine, and starts at the pyloric orifice of the stomach, on the left side of the body of the first lumbar vertebra. It is a C-shaped organ, closely applied to the back wall of the abdomen. It starts at the level of the first lumbar vertebra, passes downwards and to the right as low down as the third lumbar vertebra and, turning upwards, ends on the left side of the body of the second lumbar vertebra at the duodeno-jejunal flexure, being the point at which the jejunum, or second part of the small intestine, begins.

In the curve of the duodenum lies the head of the pancreas, the body passing to the left between the two ends. The duodenum lies on the vena cava and aorta and the two psoas muscles. It touches the right kidney, and is covered by coils of small intestine.

On the inner surface of the duodenum just behind the pylorus is seen a small prominence-the bile papilla-on which is the common opening of the bile and pancreatic ducts.

Jejunum

The Jejunum is the second part of the small intestine, and the Ileum is the third part. They measure about 20 feet together, and start at the duodeno-jejunal flexure, ending by the junction of the ileum with the large intestine at the colic valve.

The coils of the small intestine lie in the abdominal cavity, varying considerably in their disposition, as they are in continual movement.

There is very little difference between the jejunum and ileum, and the transition of one to the other is gradual. The ileum is smaller in diameter, has fewer valvulae conniventes and a larger number of Peyer's patches than the jejunum.

Large Intestine

The Large Intestine consists of several parts-the cascum, ascending, transverse, and descending colons, sigmoid flexure of colon, and rectum.

The structure is similar to that of the small intestine except that it has no villi and the longitudinal coat of muscle consists of three longitudinal bands, so that the organ has a sacculated appearance.

Small processes of peritoneum distended with fat called appendices epiploicae hang from the outer wall of the large intestine.