The Peritoneum is a serous sac which lines the abdominal cavity and invests all the structures. It is a thin membrane, secreting sufficient fluid to keep the whole surface lubricated and to enable the organs to move easily over one another. The peritoneum is really one sac, and its arrangement is very complicated, as it invests all the organs and connects them with the abdominal wall.

The connections of the organs to the abdominal wall and to one another are called by three different names :

Omenta - folds of peritoneum connecting the stomach with any other organ.

Mesenteries - folds of peritoneum connecting the intestines with the abdominal wall.

Ligaments - folds of peritoneum connecting organs (not parts of the alimentary canal) to one another or to the abdominal wall.

Following the peritoneum up the anterior abdominal wall, it passes over the under surface of the diaphragm; from the posterior edge of that it is reflected back over the upper surface of the liver. It then turns round the anterior edge of the liver and, forming the anterior fold of the small omentum, meets the stomach; it covers the anterior surface of the stomach and hangs down in a large fold called the great omentum. The posterior fold of the great omentum passes up over the posterior surface of the colon, where it is carried by the vessels to the posterior abdominal wall, and is carried off that, by the vessels, in a fan-shaped process- the mesentery- which invests the small intestine. After passing round the small intestine it again goes back to the posterior abdominal wall, where it passes down over the rectum and the upper part of the pelvic organs to the anterior abdominal wall. This constitutes the great sac of the peritoneum. The small sac covers the posterior half of the under surface of the liver, forms the posterior fold of the small omentum, and covers the posterior surface of the stomach; it then forms a fold inside that of the great omentum, and passes up over the anterior surface of the colon, back to the posterior abdominal wall.

Fig. 46. - The Peritoneum.