The membrane enclosing the contents of the abdominal cavity. The parietal peritoneum lines the walls of the abdomen and the visceral peritoneum covers the abdominal organs. The two are continuous with one another at the back of the abdomen and form a complicated closed sac (see MESENTERY; OMENTUM). A small amount of fluid is always present to lubricate the membrane, while a large amount collects in conditions associated with OEDEMA or in PERITONITIS.