520 



SEROUS AND SYNOVIAL MEMBRANES. 



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 in fig. 400. 



The layer of epithelium 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 from those in which the epi- 

 thelium of the serous membranes 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, 

 where 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, 

 winch 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 ligam en- 

 turn 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 



O O O 



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 rnucosum, the cells are 

 of a similar appearance to those of the general 

 surface of the membrane, though they seem 

 rather more delicate and transparent. 



The projecting edge of the so-called alar 

 ligaments offers still more marked characters. 

 Owing to the congestion of its vessels from 



S O 



some unknown cause, it is frequently seen 

 after death of a bright red colour, its surface 

 is minutely rough or velvety, and its consist- 

 ence soft or almost pulpy. On examining it 

 with the microscope, many minute and villus- 

 like processes are seen studding its border, 

 and directed backwards towards the commis- 

 sure of the femoral and tibial articular sur- 

 faces. These processes appear to consist 

 chiefly or entirely of two structures, viz. 

 bloodvessels and cells. The vessels are nu- 

 merous long tortuous capillaries, which pass 

 to the margin of the villus, and then, taking 

 an arched or looped course, return upon 

 themselves, and pass, with few anastomoses, 

 into the general plexus of the fold. The cells, 

 equally with the vessels, resemble those al- 

 ready described as existing at the border of 

 the articular cartilage. They are of various 

 sizes, the more numerous and larger ones are 

 spherical, transparent, and contain a tolerably 

 large nucleus : they are distended with fluid, 

 and the slightest pressure on their singularly 

 delicate cell-wall bursts the cell, and causes 

 the fluid to exude. In this condition, the 

 action of the surrounding water seems to 

 impress on it something like a partial coagu- 

 lation, giving it a mottled or minutely granular 

 appearance. 



The smaller cells exhibit the same shape 

 and general appearances, except that the 

 nucleus is proportionally larger ; a few cyto- 

 blasts are also present, and a granular blastema 

 completes the covering of the vessels. One 

 would fancy this to be a favourable situation 

 for verifying the existence of a basement 

 membrane, did such a structure exist here; 

 but I have been unable to detect it. On the 

 contrary, I have often seen the curved border 

 of a large cell seated directly on a capillary, 

 the dark line of the wall of this tube alone 

 separating its cavity from the delicate sphere 

 in contact with it. 



Fig. 401. represents such a villus-shaped 

 process. 



The relation of the synovial membranes to 

 the diarthrodial cartilages, or the question of 

 " Whether the membrane is continued over 

 the articular surface of the cartilages, or not?" 

 has been long a matter of dispute among 

 anatomists. But a resume of the history of 

 this discussion having been already given in 

 an earlier part of the work, the reader is 



