After the intestines return to the abdominal cavity, some portions press against the posterior abdominal wall and their mesenteries fuse with the parietal peritoneum. As a result, the duodenum, pancreas, ascending and descending colons adhere to the posterior abdominal wall and become secondarily retroperitoneal. The fusion of their visceral peritoneum and mesenteries with the parietal peritoneum of the posterior abdominal wall is called fusion fascia, and it is a relatively avascular plane that can be easily cleft during surgical procedures.