SYNOVIAL MEMBRANES. - The synovial membranes are structures exceedingly analo gous to the preceding, and consist of a layer of cell-growth, which covers the inner surface of the ligaments that connect the different segments of the skeleton in the diarthrodial joints, and which thus partially lines the " cavity " or interior of these articulations.
They have been usually described as re sembling the burso mucosa' both in the nature and consistence of their secretion, and in their constant adherence to the morphological character of a shut sac ; while the absence of epithelium predicated of these bursx, has been laid down as the chief anatomical dis tinction between the tvi.o structures. And, on the other hand, they have been likened to the serous menzbranes by the Common pos session of a tesselated epithelium, and by their continuity over the whole surface of the cavity and its contents ; while they have been severed from them by the difference in the composition and consistence of their secretion,—a viscid alkaline fluid, instead of a more limpid and neutral one. Most of these statements can only be received with some modification.
In the following sketch, such details as are more or less common to the synovial mem branes in general will chiefly be treated of. For a description of their more salient pecu liarities in the different joints, the reader is referred to the articles headed with the names of the several articulations.
The epithelium of these structures presents characters which afford some grounds for distinguishing it both from that of the bursal and serous membranes. It forms, for the most part, but one layer, the forms of the constituent cells of which vary to the same extent as those witnessed in the burs. But the broad, squamous, polygonal epithelia are coniparatively rare ; and in by far the larger extent of its surface, the predominant shape is that of a slightly flattened spheroidal, or oval, or somewhat angular cell, such as the majority of those represented in fig. 39P, in The vessels of the membrane are exceedingly numerous, and its capillaries form a horizontal plexus, which ramifies immediately beneath the epithelium in the areolar tissue just men tioned. The great vascularity of the tissue has long been known, but the capillaries are not only very numerous, but offer a much more remarkable peculiarity.* They are
greatly increased in their length, so as to be everywhere extremely tortuous, and some times this tortuosity almost amounts to a spiral disposition. On looking at the broad surface of welbinjected specimens, an exag geration of this disposition here and there, gives rise to small patches of tortuous capil laries ; but the arrangement is clearly a general one, and extends, in some degree, to every in dividual capillary of the net-work. But though the length of these vessels in a given space is thus greatly augmented, the frequency of their some of' which are seen decussations of two convex outlines, caused by the margin of one cell slightly overlapping that of its neighbour. Acetic acid exerts an unusual effect upon the cell-membrane, swelling up its outline very much before dissolving or rupturing it; an appearance which obtains in the more flat tened and polygonal epithelia of the serous membranes, but, so far as I have seen, in a much smaller degree. Like those of the bursw, they are firmly attached to the sub jacent tissue, and possess little mutual ad hesion ; though here and there a cluster of two or three more polygonal than usual may be found. Cytoblasts are rare, the cells ap pearing to be completed by the addition of the outer membrane when yet extremely small, (jig. 399. b.) All these peculiarities might perhaps be generalized in the statement, that the cells which cover the general surface of these membranes are in a younger and more active stage of cell-life than those of the bursae. And a slight yet perceptible difference in the same respect has been already indicated as existing between the subcutaneous and sub tendinous members of this class of structures. Immediately beneath these cells lies a stratum of looser areolar tissue, which connects the membrane with the inner aspect of the liga ments of the joint. It includes little of the yellow fibrous tissue, and its meshes are comparatively few and close : exteriorly, they unite, by a gradation of structure, with the dense white fibrous tissue of which the liga ments are composed.