Peritoneum (Gr. Peritoneum 1300168 around, and Peritoneum 1300169 to stretch), the thin, transparent serous membrane which lines the abdominal cavity of man and vertebrates, reflected upon most of its contained organs, and more or less completely enveloping them, and keeping them in place by its folds and prolongations. Like other serous membranes, it is a closed sac, covering but not containing the organs in its cavity; its internal surface, in contact with itself, is smooth and shining, moistened by a serous fluid which permits the natural movements of the organs upon each other. The double folds which, after embracing the small intestine, extend backward to the spinal column, constitute the mesentery. They form a kind of membranous attachment, by which this part of the alimentary canal is held in place, and which yet allows the necessary motions of each portion. Between the two layers of the mesentery are included the blood vessels, nerves, and lymphatic glands and vessels belonging to the small intestine. The folds of peritoneum which embrace the colon and rectum are called respectively the mesocolon and mesorectum.

The double membranous fold, prolonged like an apron from the convexity of the stomach and colon, and floating free over and in front of the intestines, is the omentum or epiploon; it is supplied with numerous vessels, and is more or less charged with fat; it serves to keep the intestines in place, and to protect them and the vessels from external injury. In the male foetus the peritoneum sends a prolongation which accompanies the testis in its descent and becomes the tunica vaginalis, which is afterward shut off from the general peritoneal cavity, forming a closed sac of its own. The cavity of the tunica vaginalis, thus formed, encloses the testicle, in the same manner as the remainder of the peritoneum encloses the organs of the abdomen. Folds of the peritoneum also form the suspensory and lateral ligaments of the liver, and the broad ligaments of the uterus. The membrane is liable to common acute inflammation. (See Peritonitis).