Secondary retroperitoneal position

The duodenal loop positions itself to the right, onto the abdominal wall. The mesoduodenum fuses with the peritoneum of the posterior abdominal wall. The merged layers of peritoneum disappear. Thereafter, most of the duodenum becomes "secondarily retroperitoneal". The pancreas and the end part of the bile duct also become affixed to the posterior abdominal wall and so become retroperitoneal. The mesentery of the midgut does not disappear; it extends from the duodenojejunal flexure to the ileocaecal valve ( L2 - ileo-sacral articulation). The area where the mesentery is fixed to the dorsal abdominal wall is called the root of the mesentery. The caecum generally grows in a caudal direction and ends up in the right iliac fossa. Most of the ascending colon is affixed to the dorsal abdominal wall. The mesentery of the caecum and the ascending colon fuse for the most part with the peritoneum of the right posterior abdominal wall in such a way that the mesentery disappears from the site of fusion. Thus, caecum and ascending colon become secondarily retroperitoneal. The transverse colon keeps its mesentery (transverse mesocolon). The descending colon becomes secondarily retroperitoneal and the sigmoid colon remains intraperitoneal.