The =peritoneum= is a thin transparent sheet of connective tissue supporting on its surface a layer of flattened epithelial cells, the peritoneal epithelium. It forms a sac which lines the entire abdominal cavity. This sac is closed in the male; in the female, however, it communicates with the exterior through the uterine (or Fallopian) tubes and uteri. All the organs of the abdominal cavity are outside the sac. In the course of their development these organs have encroached on the peritoneal sac. Each has grown against the outer wall of the sac to a greater or less extent and has forced a part of this wall ahead of it into the cavity. In some cases the encroachment has gone so far that the organ in question lies apparently within the peritoneal cavity, suspended from the wall of the sac by a fold of that wall. The wall may thus be divided into three portions. One of these, the =parietal= layer, lines the wall of the body cavity. The second (the =mesentery= in case of the alimentary canal, or a =ligament= in the case of another organ) suspends the organ from the body wall. Between the layers of each mesentery or ligament blood-vessels may pass to the organs. The third portion or =visceral= layer covers the organ in question, forming its =serous= covering. The reflections of the peritoneum to form the mesenteries and ligaments may be thus described: Caudally the peritoneum covers the whole surface of the bladder and is reflected from its ventral wall to the linea alba as the =suspensory ligament= of the bladder. Farther craniad the peritoneum suspends the rectum and colon from the middorsal line, forming the =mesorectum= and =mesocolon=. The mesocolon continues craniad to the level of the caudal end of the right kidney and is broadest at its cranial end. At this end the mesocolon passes into the =mesentery= proper, which suspends the small intestine and is very broad and much folded. Its dorsal border is attached to the median line opposite the caudal end of the kidneys and is very short compared to its very long ventral or intestinal border. Toward the cranial end the mesentery of the jejunum passes gradually into the very much shorter duodenal mesentery. This is drawn out at the caudal end of the duodenum into a fold, the =duodenorenal= ligament which attaches the duodenum to the kidney.