Inflammation of the lining of the abdominal cavity. Before antibiotics, people would die from peritonitis if an inflamed appendix burst. Indications of peritonitis are called "peritoneal signs": tender abdomen, rebound pain (pain when manual pressure released from examining abdomen), board-like rigidity of abdominal muscles, no bowel sounds (gurgles). The peritoneal membrane is very sensitive to exposure to foreign substances. Contact with blood, bile, urine, pus will cause peritoneal signs.