MORBID ANATOMY OF SEROUS AND SY NI:MAL MEMBRANES. - The following sketch of the diseased appearances of these tissues is necessarily limited to their more general features. At present, it is scarcely possible to discriminate between the very analogous pa thological conditions of the two classes of serous and synovial membrane ; although it is probable that an advance of knowledge will at no distant date enable us to do so. And even where the distinctions of appearances are sufficiently palpable, our ignorance of their general nature allows few inferences to be drawn from these varieties.
Thus, the remarkable immunity from me chanically-produced effusions which the syno vial membranes enjoy is little understood, although one may perhaps doubt whether it is quite so complete as it is generally sup posed to be. The only conjecture that seems at all probable is, that the nature and activity of the cell-growth which covers their surface may have some relation to the diffi culty with which such fluids transude. So, also, the comparative infrequency of adhesion in their inflainmations is, at present, a vague fact, the cause of which is unknown ;—it may either be referred to an explanation similar to the preceding, or may, as Profes sors Todd and Bowman suggest*, depend upon the presence of a viscid secretion in their interior.
Serous or dropsical effusions. — One of the most frequent of the morbid appearances seen iu these tissues is the presence of a serum-like fluid in their cavity. It occurs in a very large number of deaths from various diseases. In most instances, however, the serous membrane only shares in a dropsy which is common to other structures, and especially affects the areolar tissue. Thus, for instance, where death has resulted from some mediate or immediate obstruction to the passage of blood through the right cavi ties of the heart, and has been preceded for some time by general anasarca, it is usual to find a considerable quantity of fluid occupy ing the pleura, peritoneum, and other serous membranes. In other diseases, as in cirrhosis, the serous effusion is not only a more direct result of a greater venous obstruction, but it also assumes a higher import than in the previous instance, and becomes both of earlier occurrence in the history of the dis order, and of weightier influence upon its termination. Here, an accompanying dropsy of the areolar tissue is less frequent and prominent, but it still generally follows at a certain stage of duration and intensity : it is usually ascribed to the pressure of the dis tended belly acting upon the vena cava, and producing a secondary dropsy from the branches of the systemic circulation which join that vein.
In another class of cases, the serous effu sion is still common to the areolar tissue and the serous membranes, but it arises from a different cause ; one which is no longer a mechanical impediment, but a chemical altera tion. The dropsy of chlorosis is a good example of this species of effusion and to it may probably be also referred tilt seen in the latter stage of phthisis and other ex hausting disorders.. Analysis shows, that in these ancenziee the blood is rendered much poorer by the loss of a considerable propor tion of its albumen, and the serum of the thus diluted fluitl possesses a greater tendency to transude the membranous walls of the vessels, and pervades the surrounding struc tures in an undue quantity. Below a certain percentage of albumen, Andral affirms the occurrence of dropsy to be tolerably constant.
To these two classes may be added a third, in which serum is found in these structures without any sign or symptom of its presence having been detected during life. These cases are so numerous, that, even after sub tracting a considerable number as possibly due to neglect or difficulty of recognition during life, a large number still remain, in which the effusion may be fairly presumed to have taken place after death. And irt many instances, they are not only affected by gravi tation, but, like the very analogous condition of the areolar tissue, their occurrence seems to be favoured by it. Yet, as such appear., ances are absent from a large majority of post-mortem inspections, it will follow, that the effusion of this fluid is to be ascribed, at least in part, to some conditions other than mere gravity. These are probably similar to the circumstances which conduce to the pro duction of the preceding class of effitsions, a deficiency of the albuminous constituent in the blood, or, with lesser likelihood, the condition of the walls of the containing vessels themselves. They thus appear to be due to both a mechanical and chemical affec tion of the blood, and so seem to offer an union of the two causes to which the pre ceding classes have been severally ascribed.