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The lesser omentum is a portion of the peritoneum, the membrane encapsulating most of the organs within the abdominal cavity, that is found between the liver and the stomach. This structure is sometimes known by several other names, including the gastrohepatic omentum and small omentum. The double-layered membrane is made up of both fibrous connective tissue, similar to a ligament, and mesothelium, or a thin layer of flat membranous cells. Though it is made up of two layers, it is little more than a narrow sheet of tissue stretching between the two organs.
In addition to surrounding the collective organs of the abdominal cavity, the peritoneum also forms a layer around each individual organ. The lesser omentum is simply a continuation of such a layer, with a single membrane from the front and back sides of the stomach converging to form a double membrane that stretches toward the underside of the liver. If the torso were split into upper and lower halves and viewed in cross-section, the lesser omentum would appear as a horizontal line between the stomach on one side of the abdomen and the liver on the other.
Below the liver, it arises from around the porta hepatis. Found on the inferior aspect or underside of the liver, the porta hepatis is the place where the vessels essential to liver function enter and exit the organ: the hepatic artery, portal vein, common bile duct, lymph vessels, and the bundle of nerves known as the hepatic plexus. The lesser omentum wraps these vessels, which angle downward out of the underside of the liver, from in front and behind like a dried flower is pressed between plastic and page in a photo album. Immediately to the outside of these vessels on their lateral side, the two layers of omentum join together to form an unattached border known as the free margin.
In the opposite direction of the free margin, the two layers stretch thinly beyond the porta hepatis toward the stomach’s lower surface. Approaching the stomach from its medial side, or that which is nearest to the midline of the body, the layers separate to pass in front of and behind the upper duodenum, or the topmost segment of the small intestine that exits the stomach and angles immediately downward. They continue until they reach the lesser curvature of the stomach, the curve of the organ along its top-inside surface. Here the two layers of the lesser omentum begin to wrap the stomach as a whole and as such become that organ’s peritoneal membrane.