The greater sac is the main cavity; the lesser sac lies behind the stomach
Mnemonic
The epiploic (omental) foramen of Winslow is the only communication between the two sacs, and its boundaries are worth knowing precisely:
- Anterior - free edge of the lesser omentum, containing the portal triad
- Posterior - inferior vena cava
- Superior - caudate lobe of the liver
- Inferior - first part of the duodenum
“Portal triad in front, cava behind, caudate above, duodenum below.”
A finger passed through the foramen can compress the portal triad against the thumb, which is the Pringle manoeuvre used to control bleeding from the liver.
The lesser sac lies behind the stomach, which is why a pancreatic pseudocyst collects there and why posterior gastric wall perforation drains into it rather than into the general peritoneal cavity.
Expansion
- Greater sac - the main peritoneal cavity, entered as soon as the abdomen is opened
- Lesser sac (omental bursa) - a diverticulum lying behind the stomach and the lesser omentum, in front of the pancreas
They communicate only through the epiploic foramen (of Winslow).
The lesser sac exists because the stomach rotates during development, carrying the dorsal mesogastrium to the left and creating a recess behind it.
Clinically it matters because collections can be trapped there. A pancreatic pseudocyst typically forms in the lesser sac, since the pancreas lies in its posterior wall, and a perforated posterior gastric ulcer leaks into it rather than into the general cavity, which is why signs may initially be muted.