The parietal peritoneum was previously partially removed from the posterior abdominal wall ( see Laboratory Thirty-Nine). Beginning from the lateral border of the ascending colon, the descending colon, and the sigmoid colon, free the organs from the peritoneum of the posterior abdominal wall. Lift these organs with their attached vessels and nerves from their site of attachment, and reflect them toward the midline ("mobilize the colon"). This procedure reestablishes an embryonic plane in which the vessels and nerves pass. The retroperitoneal space and posterior abdominal wall on each side of the vertebral column can be accessed.