Describe the lesser sac, mesenteries, peritoneal ligaments, and retroperitoneal organs.

b) The lesser sac (omental bursa) is a peritoneal pouch located posterior to the stomach and lesser omentum, and anterior to the pancreas and parts of the duodenum. It communicates with the greater sac via the epiploic foramen (foramen of Winslow). c) A mesentery is a double layer of peritoneum that suspends an organ from the posterior abdominal wall, allowing it to move while providing a conduit for blood vessels, nerves, and lymphatics to reach the organ. d) A peritoneal ligament is a double layer of peritoneum that connects an organ to another organ or to the abdominal wall. It typically contains blood vessels, nerves, or ducts. e) Three mesenteries apart from that of the small intestines are: • Transverse mesocolon (suspends the transverse colon) • Sigmoid mesocolon (suspends the sigmoid colon) • Mesoappendix (suspends the appendix) f) Retroperitoneal means located behind the peritoneum. Organs that are retroperitoneal are not suspended by a mesentery and lie against the posterior abdominal wall, covered by peritoneum only on their anterior surface. g) Primarily retroperitoneal organs are those that developed and remained outside the peritoneal cavity throughout development, never having a mesentery. Examples include the kidneys, adrenal glands, ureters, aorta, and inferior vena cava. Secondarily retroperitoneal organs are those that initially developed within the peritoneal cavity with a mesentery but later fused with the posterior abdominal wall during development, losing their mesentery and becoming retroperitoneal. Examples include the pancreas, duodenum (except for the first part), and ascending and descending colon.