peritoneum (pĕrətənēˈəm) [key], multilayered membrane which lines the abdominal cavity, and supports and covers the organs within it. The part of the membrane that lines the abdominal cavity is called the parietal peritoneum. The portion that covers the internal organs, or viscera, is known as the visceral peritoneum and forms the outer layer (serosa) of most of the intestinal tract. The supportive peritoneum forms sheets of greatly modified membranes called mesenteries. These tissues hold the organs of the digestive tract in position and convey nerves, blood vessels, and lymphatic ducts to the viscera. The space between the visceral and parietal membranes contains a watery fluid that permits the abdominal organs to slide freely against the abdominal wall. A ruptured appendix can lead to inflammation of the peritoneum, a condition known as peritonitis.