The lesser sac, also known as the omental bursa, is a part of the peritoneal cavity that is formed by the lesser and greater omentum. Usually found in mammals, it is connected with the greater sac via the omental foramen or Foramen of Winslow. In mammals, it is common for the lesser sac to contain considerable amounts of fat.

Anterior margin

listed from the top-to-bottom margin: Caudate lobe of the liver, lesser omentum, stomach, gastrocolic ligament

Lateral margin

listed from the most anterior to the most posterior margin: Gastrosplenic ligament, spleen, Lienorenal ligament

Posterior margin

Left kidney and adrenal gland, pancreas

Inferior margin

Greater omentum

Superior margin

Liver^[1]

If any of the marginal structures rupture their contents could leak into the lesser sac. If the stomach were to rupture on its anterior side though the leak would collect in the greater sac.^{[citation needed]}

The lesser sac is formed during embryogenesis from an infolding of the greater omentum. The open end of the infolding, known as the omental foramen is usually close to the stomach.^{[citation needed]}

• Schematic figure of the bursa omentalis, etc. Human embryo of eight weeks.
• Diagrams to illustrate the development of the greater omentum and transverse mesocolon
• Omental bursa

• Terms for anatomical location
• Greater sac
• Omental foramen (Epiploic foramen, Foramen of Winslow)
• Lesser omentum
• Greater omentum
• Peritoneum