In the chapter on Embryology we have seen how the dorsal and ventral mesenteries are formed by the meeting of the hypomeres dorsal and ventral to the digestive tract. These mesenteries consist therefore of two layers of mesoderm continuous with the parietal peritoneum, lining the body cavity, and the visceral peritoneum (serosa), covering the digestive tract. Thus they serve both as attachments of the digestive tract to the body wall and as bridges for the passage of blood vessels and nerves, which run between the two otherwise closely applied layers. During embryonic development the mesenteries undergo many changes including the dropping out of most of the ventral mesentery and modifications due to the great elongation of the intestine as well as the invasion of the mesenteries by such structures as liver, pancreas, and spleen.
With the differentiation of the abdominal digestive tract into stomach, small intestine, colon, and rectum, the corresponding parts of the dorsal mesentery become known as mesogaster, mesentery proper, mesocolon, and mesorectum, respectively. When the liver grows into the anterior remnant of the ventral mesentery, the part of the latter between liver and ventral body wall is called the falciform ligament (ventral ligament of the liver), while the part between the liver and the digestive tract is the lesser omentum in which run the bile duct, hepatic portal vein, and other smaller blood vessels. Wrhere the liver has pushed the two mesenteric layers apart to lie between them they become the serosa, peritoneal covering, of this organ. A posterior remnant of the ventral mesentery also persists as the ventral ligament of the bladder, extending from that organ to the ventral body wall.
In mammals the mesogaster, which it will be remembered is composed of two layers of peritoneal tissue, becomes greatly elongated until it extends from the stomach posteriorly, in between ventral body wall and the coils of the intestine, to serve as a sort of ventral protective blanket for the viscera (Fig. 272). This folded portion of the mesogaster, known as the greater omentum, is a bag the walls of which are two layers thick. Where these walls become closely applied to one another in the posterior part of the sac, this omentum is an apron four layers thick. The omentum is also a storage organ for extensive fat deposits, laid down between what were originally the two mesenteric layers.