The duodenum, so called on account of its length (being about twelve inches long), is, unlike the other parts of the small intestine, very definite in position and extent. It is that part which is not suspended by the mesentery. It is, further, the most fixed as well as the widest part of the small intestine, measuring one and one-half to two inches 1 in diameter. It has a curved shape, somewhat resembling that of a horseshoe. It surrounds the pancreas and is divided into four parts:

1. The superior horizontal portion (pars horizontalis superior) begins at the pylorus, lying at the level of the first lumbar vertebra, and runs slightly upward and backward toward the right until it reaches the right side of the vertebral column. It ends at the neck of the gall bladder, and is the most movable of the four portions. It is covered by the two layers of the peritoneum which are continued from the stomach, and by these it is completely surrounded. Above it lie the liver (quadrate lobe) and the gall bladder, below it is the pancreas, and behind it are the common bile duct and hepatic vessels.

2. The descending portion of the duodenum, beginning at the neck of the gall bladder, is about twice as long as the first portion, and runs almost vertically to the second or third lumbar vertebra. It lies to the right of the lumbar vertebrae, and touches the right kidney. In front of it and crossing it almost at a right angle, runs the transverse colon. It is more fixed than the first portion. On its left side is the pancreas, and the common bile duct a little more posteriorly. Into this part of the bowel, and at its inner and back part, but four inches from the pylorus, the common bile duct and pancreatic duct enter. The portion at which these ducts enter, occasionally forms a small sinus (diverticulum or ampulla Vateri).

3. The third part or the transverse portion is the longest, measuring about five inches. It extends from the base of the second or third lumbar vertebra on the right side obliquely across the spine to the upper part of the left side, ascending a little on its way. In front of it is found the lower layer of the transverse mesocolon. The superior mesenteric vessels cross this part of the duodenum, running between it and the pancreas in order to reach the mesentery. This portion is in relation with the pancreas and superior mesenteric artery above, with the vena cava, aorta, and crura of the diaphragm behind. It is the most fixed portion of the duodenum.

The Intestine, as Seen from the Front after Removing the Omentum

Fig. 1. - The Intestine, as Seen from the Front after Removing the Omentum (Testut). 1, Abdominal wall; 2, wall of the thorax; 3, oesophagus; 3', cardia; 4, stomach; 4', pylorus; 5, duodenum; 6, pancreas; 7, liver; 8, gall bladder; 9, gastrohepatic ligament: 10, right kidney and its suprarenal capsule; 11. small intestine; 12, terminal portion of the ileum; 13, escnm; 13', its appendix; 14. ascending colon; 15, transverse coIon; 16, descending colon; 17, ileopelvic colon; 18, bladder; 19, parietal peritoneum; 20, spleen: 21. diaphragm: 22, thoracic aorta.

4. The fourth part of the duodenum or second ascending portion ascends vertically at the left side of the spine. It is about one inch long and forms the end of the duodenum. It is firmly fixed in its place by the musculus suspensorius duodeni, the latter being the name of the fibrous band, containing some plain muscular fibres which descend to the vertical part of the duodenum from the left crus of the diaphragm and the tissues about the coeliac axis. It terminates at this point in the jejunum, forming the flexura duodenojejunal at a place situated to the left of the second lumbar vertebra.