Peritoneum, the closed serous sac which enfolds the abdominal organs. The visceral layer of the peritoneum is reflected over, and forms the outer covering of the several abdominal viscera; this layer is continuous with the parietal layer of the peritoneum which underlies the external abdominal wall, and between the two layers is enclosed what is known as the cavity of the peritoneum. The folds of peritoneum reflected over the intestines, and which serve to attach them to the back part of the abdomen, enclose certain blood-vessels and nerves, forming the structure which is called mesentery. Other connecting folds of the peritoneum are attached to other abdominal organs, and form the outer covering of the ligaments of those organs. A peculiar fold of peritoneum, which is spread over a portion of the intestines like an apron, is called the great omentum.