This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The duodenum is the thickest, widest, and most fixed portion of the small intestine. Its diameter is from 3.75 cm. to 5 cm. (1 1/2 to 2 in.) and its muscular and mucous coats are thicker than those of the jejunum or ileum. It also possesses in its upper half the glands of Brunner (glandulae duodenales) in the submucous coat. It is thus seen that in its structure it resembles more the stomach than the intestine and, like the stomach, is especially prone to ulcer. While carcinoma frequently originates at the pylorus and extends to and involves other parts of the stomach, it does not tend to involve the duodenum. This is probably due to the lymph stream from the pylorus running toward the stomach and away from the duodenum. The duodenum is also of interest in consequence of its intimate relation to the biliary passages and gall-bladder as well as to the pancreas and its ducts. Inflammations, such as accompany gall-stones, frequently give rise to adhesions, to relieve which operations are performed. The second portion of the duodenum is sometimes opened in order to extract a biliary calculus impacted in the ampulla of Vater at the mouth of the common bile-duct. The upper portion of the duodenum in Finney's operation for pyloroplasty is slit down from the strictured pylorus and sewed to a corresponding slit in the stomach, thus making a large communication between the duodenum and the stomach and eliminating the stricture.
In shape the duodenum resembles a horseshoe. It begins on the right side of the body of the first lumbar vertebra and ends on the left side of the body of the second lumbar vertebra. At its commencement it is suspended from the liver by the duodenohepatic ligament, which is the free edge of the gastrohepatic omentum in which run the hepatic artery, portal vein, and common bile-duct.
Fig. 419. - The duodenum, showing its course and relation to the surrounding organs.
The duodenum is composed of four portions. The first portion (superior) begins at the pylorus and ends at the neck of the gall-bladder. It is about 5 cm. (2 in.) long, and runs backward along the body of the first lumbar vertebra. The second portion (descending) is about 10 cm. (4 in.) long, and runs down the right side of the bodies of the lumbar vertebrae to the lower border of the third. The third portion (variously called ascending, transverse, or inferior) runs diagonally upward across the body of the third lumbar vertebra to its left side and then the fourth portion ascends to the left side of the second, where it takes a sharp turn and is continued as the jejunum (Fig. 419).
First portion: Above and in front are the quadrate lobe of the liver and the gall-bladder; below is the pancreas; and behind, from left to right, lie the gastroduodenal artery, the portal vein, the common bile-duct, and the vena cava.
Second portion: In front is the liver, the neck of the gall-bladder, and the transverse colon. Behind are the renal vessels, ureter, right kidney, and psoas muscle. To its inner side lie the pancreas and vena cava. The common bile-duct runs on the inner side between the duodenum and the pancreas; at the middle of this portion of the duodenum the bile-duct joins with the pancreatic duct to empty into the duodenum through the ampulla of Vater, about 10 cm. (4 in.) from the pylorus.
Third portion: In front are the superior mesenteric artery and root of the mesentery; behind lie the vena cava, aorta, and left psoas muscle. Above, it lies in contact with the pancreas. The termination of the duodenum is usually on the left of the aorta, but Dwight (Journal of Anatomy and Physiology, vol. xxxi, p. 576) in fifty-four cases found it twenty-six times on the right of the aorta until just before its terminal flexure. It was wholly on the right side six times, in front of the aorta eleven times, and had crossed the aorta eleven times.
First part: The pyloric half is almost completely covered by peritoneum, but the distal half only on its anterior surface. Second part: No peritoneum on its inner and posterior surfaces, and only on its outer and anterior where not covered by the transverse colon. Third and fourth parts: The anterior and left sides are covered by peritoneum except where crossed by the root of the mesentery and superior mesentric vessels.