The duodenum curves in a C around the head of the pancreas and is 10 in (25 cm) long. At its origin from the pylorus it is completely covered with peritoneum for about 1in (2.5 cm), but then becomes a retroperitoneal organ, only partially covered by serous membrane.
Relations (Figs 57, 58)
For descriptive purposes, the duodenum is divided into four sections.
The first part (2 in (5 cm)) ascends from the gastroduodenal junction, overlapped by the liver and gall-bladder. Immediately posterior to it lie the portal vein, common bile duct and gastroduodenal artery which separate it from the inferior vena cava.
The second part (3 in (7.5 cm)) descends in a curve around the head of the pancreas. It is crossed by the transverse colon and lies on the right kidney and ureter. Half-way along, its posteromedial aspect enters the common opening of the bile duct and main pancreatic duct (of Wirsung) on to an eminence called the duodenal papilla. This common opening is guarded by the sphincter of Oddi. The subsidiary pancreatic duct (of Santorini) opens into the duodenum a little above the papilla.
The third part (4in (10 cm)) runs transversely to the left, crossing the inferior vena cava, the aorta and the third lumbar vertebra. It is itself crossed anteriorly by the root of the mesentery and the superior mesenteric vessels. Its upper border hugs the pancreatic head.
The fourth part (1 in (2.5 cm)) ascends upwards and to the left to end at the duodenojejunal junction. It is surprisingly easy for the surgeon to confuse this with the ileocaecal junction, a mistake which may be disastrous. He confirms the identity of the duodenal termination by the presence of the suspensory ligament of Treitz, which is a well-marked peritoneal fold descending from the right crus of the diaphragm to the duodenal termination, and by visualizing the inferior mesenteric vein which descends from behind the pancreas immediately to the left of the duodenojejunal junction.
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