=The Peritoneum and the Mesentery.=—The abdominal and pelvic cavities are lined by the peritoneum. Like all serous membranes this is composed of a parietal and a visceral portion, which together form a complete sac, with the organs it covers situated on the outer side. The peritoneum like other serous membranes consists of a mesothelial and a submesothelial portion, the cells of the former being arranged in a single layer. Since a serous membrane is so arranged as to line a closed cavity, and at the same time to cover its contents, it follows that the entire membrane must be a closed sac, the mesothelial layer being on the inside; such a sac is called a serous sac, or cavity. Synovial membranes are also regarded as a variety of serous membranes. The fold, or layer, of the membrane which lines the cavity is called the parietal, that which covers the greater part of the organs contained therein is called the visceral portion; the two surfaces contacting, and gliding upon each other, are lubricated by a fluid secretion contained in the sac; hence one use of these membranes is to prevent friction between the walls of cavities and the organs contained therein. Serous membranes line the abdominal cavity, pericardium, cavities of the heart and is continuous throughout the vascular structures. [Illustration: FIG. 39.—A transverse section of body of hen through 15, Fig. 34. _R_, Right side. _L_, Left side. 1, Spinal cord. 2, Esophagus. 3, Trachea. 4, Skin. 5, Pectoral muscles. ] The serous membrane besides covering the external surface of the viscera, double folds pass from one organ to another, or from an organ to the parietes of the cavity. These double folds of the peritoneum are known as ligaments, or as mesenteries. In ligaments the two folds are strengthened by an interposed layer of fibro-elastic tissue. A _mesentery_ is a broad, double fold of peritoneum, attached to the abdominal parietes above, and containing a portion of the intestine in its free or remote extremity. Between its folds we find blood-vessels, nerves, and lymphatics or lacteals, hence it permits vascular and nervous communications with the organ attached to it. The free portion of the small intestine is attached to the free margin of the mesentery. THE RELATIONS OF THE VISCERAL ORGANS OF THE DOMESTIC FOWL [Illustration: FIG. 40.—Transverse section through the body of a hen through 14, Fig. 34. _R_, Right side. _L_, Left side. 1, Spinal cord. 2, Esophagus. 3, Trachea near inferior larynx. 4, Lungs. 5, Pectoral muscles. ]