394 LYMPHATIC SYSTEM. 



epithelial layer of flattened cells, similar to those lining the serous membranes, but, 

 as was shown by Hiiter, there exists on the synovial membranes no complete lining 

 of the kind. Patches of cells may, it is true, here and there be met with which 

 present an epithelioid appearance (fig. 450, B, e\ as, indeed, we know to be the 

 case in the connective tissue of other parts ; but most of the surface-cells of the 

 synovial membranes are of the irregularly-branched type (fig. 450, A), the surface of 

 the membrane between the cells and sometimes also over them being formed by the 

 ground-substance of the connective tissue, whilst here and there small blood-vessels 

 come close to the surface from subjacent parts. The cells are in many places smaller 

 than in connective tissue generally. They vary much in shape in different parts, 

 sometimes forming a network in the tissue by the anastomoses of their ramifying 

 processes, in other parts being rounded, and more closely arranged. 



The cells of the vaginal synovial membrane are often slightly elongated in the 

 direction of the axis of the tendon. 



The articular synovial membranes pass, as before said, a certain distance over the 

 cartilages of the joints. They do not, however, end abruptly, but shade off gradually 



Fig. 451. TRANSITION OF CARTILAGE-CELLS INTO CONNECTIVE-TISSUE CORPUSCLES OF SYNOVIAL MEMBRANE. 

 ABOUT 340 DIAMETERS. FROM HEAD OF METATARSAL BONE, HUMAN. (E. A. S.) 



a, ordinary cartilage cells ; b, b, with branched processes. 



into the margin of the cartilage, the fibrous tissue becoming fibro-cartilage and the 

 cells gradually losing their processes and becoming transformed into cartilage-cells 

 (fig. 451), so that it is difficult to say where the one structure begins and the other 

 ends. This portion of the synovial membrane, or of the cartilage, is known as the 

 " marginal zone ; " it is best marked around the convex heads of the bones, and is 

 especially well seen near the lower margin of the patella (Hiiter). 



The Haversian folds and fringes, at least the larger ones, agree in general 

 structure with the rest of the tissue of the synovial membrane, except that, as 

 before remarked, some of them contain fat ; their surface layer contains for the 

 most part irregularly stellate cells, except over the fat, where there is occasionally 

 to be observed a true epithelioid covering like that of a serous membrane. The 

 smaller non-vascular secondary fringes of Eainey (synovial mtti) are minute finger- 

 shaped processes projecting from the margins of the larger ones, and consist for the 

 most part of small rounded cells with granular protoplasm and but little intercellular 

 substance, enclosing a central strand of connective-tissue fibrils ; and in some cases 

 a few cartilage-cells. Some of the synovial villi are entirely made up of fibro-carti- 

 lage, being altogether destitute of the covering of rounded cells (Tillmanns). 



Vessels and lymphatics. The blood-vessels in and immediately beneath the 

 synovial membrane are numerous in most parts of the joints. They advance but a 



