The pylorus and duodenal bulb (first portion of duodenum) lie anterior to the junction of head and neck of the pancreas. The first and second portion of the duodenum passes posterolaterally and curves downward to cradle the head of the pancreas (C-loop). The second portion of the duodenum is the site of the ampulla of Vater draining the bile duct and pancreas. The third portion of the duodenum passes transversely to the left on the caudal side of the pancreatic head. The superior mesenteric vein and artery cross the third portion and in rare instances may compress the duodenum and cause obstructive symptoms. The fourth portion of the duodenum passes up to the left of the mesenteric vessels and turns sharply downward forming a knuckle beneath the tail of the pancreas. The apex of this knuckle is suspended from the lateral side of the upper lumbar vertebrae by the ligament of Treitz (dash). This must be incised to free the fourth part of the duodenum, for example in a pancreaticoduodenectomy (Whipple procedure). The peritoneal reflection around the duodenum beneath the transverse mesocolon is not the ligament of Treitz as commonly supposed.
The posterior abdominal viscera are covered by the posterior parietal peritoneum creating the retroperitoneal space. The peritoneum reflects onto the anterior abdominal organs as visceral peritoneum and the potential space between parietal and visceral layers is the peritoneal cavity. The coronary ligaments of the liver form two points laterally (right and left triangular ligaments), are continuous with the falciform ligament anteriorly, and surround the hepatic portion of the inferior vena cava and hepatic veins. The left triangular ligament lies anterior to the esophageal hiatus. When the stomach rotated to the left, the left side of the dorsal mesentery fused with the parietal peritoneum and the right side of the mesentery became the posterior surface of the omental bursa (lesser sac). The lateral boundary of the lesser sac is the gastrosplenic ligament which continues up to the esophageal hiatus as the gastrophrenic ligament. The upper recess of the lesser sac lies between the inferior vena cava and the left gastric artery. The caudal limit of the former dorsal mesentery is the hepatoduodenal ligament containing the portal vein, hepatic artery and bile duct. This layer continues as the gastrohepatic (lesser) omentum between the lesser curve of the stomach and the liver (terminating in the fissure for the ligamentum venosum-see caudal view of liver). The origin of the transverse mesocolon crosses the second portion of the duodenum and pancreas and is continuous with the root of the small bowel mesentery and the right and left colonic reflections.
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