Serous Membranes.

Serous Membranes. There are six of these membranes in the human body, two being median and single, while two are double and lateral. They are the pericardium and the peritoneum, with the two pleurae and tunicae vaginales testis. They are all closed sacs, with one exception; and each sac or continuous membrane consists of two portions—a parietal one, which lines the walls of the cavity, and a visceral or reflected one, which forms an almost complete coating or investment for the viscera contained in the cavity. The interior of the sac contains a small quantity of fluid, usually merely enough to moisten the contiguous surfaces and thereby enable them to glide easily upon each other. With regard to their structure, it is sufficient to state that they consist essentially of (1) endothelium; (2) basement membrane; (3) a stratum of areolar or cellular tissue, which constitutes the chief thickness of the membrane, and is the constituent on which its physical properties are mainly dependent. This layer is more liable to variation than the others, and one of the most common alterations is an angmentation of the yellow fibrous element, by which an increased elasticity is given to the membrane, which is thus better adapted for distention, and for a subsequent return to its original bulk. The situations in which this angmentation is found are, as Dr Brinton (Cyclopædia of Anatomy and Physiology, vol. iv. p. 524) has pointed out, in exact conformity with this view: in the peritoneum, which lines the anterior abdominal wall and covers the bladder, it attains its maximum; in the detached folds of the mesentery, in the costal pleurae, and in the suspensory ligament of the liver it is still very prominent; while on the posterior wall of the belly, and in serous membranes covering the heart, liver, &c. it is almost absent. For some of the principal serous membranes, see the articles PERICARDIUM, PERITONEUM, PLEURISY, and RESPIRATION (Vol. VIII. p. 660).—Synovial membranes (q.v.) present many points of similarity to serous membranes.