Omenta are formed from layers of visceral peritoneum folded on itself. In the abdomen, there is a lesser and a greater omentum.

Lesser omentum

The lesser omentum (which can be divided into the hepatogastric and hepatoduodenal ligaments) is formed by the anterior and posterior layers of the stomach’s visceral peritoneum folding together at the lesser curvature into a double sheet that extends towards the liver. Between the layers of the lesser omentum run several vessels and nerves, including the hepatic artery proper, the common bile duct, lymphatics, the hepatic plexus as well as some gastric arteries and veins. It delineates the anterior boundary of the lesser sac (an abdominal cavity) which can be accessed by the epiploic foramen (of Winslow).

Greater omentum

On the other hand, the greater omentum is similarly formed by the layers of stomach peritoneum folding together to form a double layer at the greater curvature and extending towards the transverse colon. Instead of forming a taut connection with the transverse colon, this double layer hangs downwards and folds on itself, forming a quadruple-layered ‘apron’ that lays on top of a large proportion of the lower abdomen. The right and left gastroepiploic arteries provide blood supply to the greater omentum.