' " inosculations does not seem to experience a corresponding increase. Their tortuous form is represented infig. 400.
The preceding description of the vascular, fibrous, and areolar constituents of synovial membrane applies only to the simplest form of that tissue, which consists of a plain flat expanse of membrane. In special joints, as well as in special parts of every joint, each of them experiences modifications deserving of notice. Over the cartilage of the articulation, for instance, all of these cease ; and deferring for the present a consideration of the ana logous structure which here supplies the place of epithelium, we come to consider the anatomy of the synovial membrane where it reaches the border of articular cartilage.
The fibrous tissue exterior to the mem brane, and with which its areolar tissue is mingled, passes to the side of the articular cartilage, and immediately becomes inextri cably interlaced with its fibrous tissue or pe richondrium. The plexus of capillaries, some what more tortuous here than on the plain surface, runs up to the edge of the cartilage, or may even advance a very short distance over it, where it is not exposed to friction during the movements of the joint. Its various branches then suddenly stop short, and each taking a looped course, returns upon itself in the same tortuous manner. This distribution is represented infig. 400.
The layer of epitheliutn offers equally re markable appearances ; a few of its particles are very slightly flattened, but most of them are spherical, and of very various sizes, of which some are extremely large. All of the larger contain a pale and rather flattened nucleus, which is in contact with a part of their inner surface. The cells are also of singular delicacy and transparency, and are, to all appearance, distended with a fluid, the refractility and colour of which closely ap proximate to that of water. The areolar tissue which forms the foundation of the membrane being diverted at this point to join with the ligaments and perichondrium, the vessels are left comparatively naked ; and so far as I have been able to make out, upon these bare capillaries the cells are seated, without the intervention of any membrane.
They thus form what is indeed a covering for the vessels (since there is no part of them upon which large or small cells or cytoblasts are not placed) ; but, as is evident from their shape only, they constitute a layer in a very different sense ftom those in which the epi thelium of the serous tnembranes does so.
In some of the more complex joints, another modification occurs, which is in many respects very similar to this, viz. distinct folds or invo lutions of synovial membrane, which project into the cavity of the joint. The best in stances of this are seen in the knee-joint, Inhere they form what are called the "mucous" and " alar ligaments." The folds which con stitute these come off horizontally from the synovial membrane in the front of the articu lation, but with a considerable interval be tween their upper and lower layers, which is filled with adipose tissue. They contain besides, a plexus of vessels, of which some, lying immediately beneath the membrane, ramify in the flexuous manner described ; while the deeper are distributed to the fat vesicles, throwing loops around each in the manner peculiar to this tissue. A very small quantity of fine areolar tissue is present, chiefly as a covering and protection to the vessels. Gradually going backwards, they lose their adipose tissue, and taper to an edge, which accurately fits into the interstice be tween the condyles of the femur and head of the tibia. Here the upper and under layers come into contact, and in the middle line pursue their way backwards as the ligamen taut mucosum, a flat, thin duplication of the membrane ; until, finally, at the anterior ter mination of the notch between the condyles, they terminate by joining the synovial cover ing and fibres of the neighbouring crucial ligament. On either side of the middle line, the process of synovial membrane terminates, by a convex margin, a little beyond the point where it ceases to contain fat : these are the " alar ligaments." On the ligamentum mucosum, the cells are of a similar appearance to those of the general surface of the membrane, though they seem rather more delicate and transparent.