SEROUS MEMBRANES. There are seven of these membranes in the human body, three being median and single, while two are dorble and lateral. They are the arach noid, the pericardium, and the peritoneum, with the two pleura; and tuniete vaginales testis. Thus they are connected, with the obvious view of facilitating motion and affording general protection, with all the most important organs in the body. They are all closed sacs, with one exception, and a reference to the articles PERICARDIUM, PERI TONEUM, and PLEURA:, will at once show the reader that each sac or continuous mem brane 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 is filled during life with a halitus or vapor, which after death condenses into a serous fluid. With regard to their structure it is sufficient to state that they corrsist•essentially of (1) epithelium (2) base ment 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 augmentation of the yellow fibrous element, by which an increased elasticity is given to the membrane, whieleis thus better adapted for distention, and for a subsequent return to its original bulk. The situations in which this augmen
tation is found are, as Dr. Brinton (Cydapceclia of Anatomy and Physiology, vol. iv. p. 524) has pointed out, in exact conformity with this view : in the peritoneum, which lines the ahterior 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, etc.. it is almost absent.
The following are the most important of time morbid changes to which these mem branes are liable. One of the most frequent of the morbid appearances seen in these structures is the presence of an excess of serous fluid in their cavity. This condition occurs in deaths from various diseases, and in general the serous membrane only shares in a dropsy which is common to other structures, and especially affects the areolar or cellular issue. When general anasarca, or dropsy of the cellular tissue, has existed for a long time, more or less dropsical effusion is usually found in the pleura; and perito neum. The inflanfmation of these structures is sufficiently described in the articles