SYNOVIA^ MEMBRANES. 41 



various parts of the body are found closed sacs, sheaths, etc., 

 which resemble synovial membranes both in structure and 

 function. Every movable joint is enveloped in a capsule 

 which is closely adherent to the edges of the articulating 

 cartilage and is even reflected upon its surface for a short 

 distance. It was formerly thought that these membranes, 

 like the serous sacs, were closed bags, with one layer 

 attached to the cartilage, and the other passing between 

 the bones so as to enclose the joint; but it is now the 

 general opinion that the cartilage which encrusts the articu- 

 lating extremities of the bones, though bathed in synovial 

 fluid, is not itself covered by a membrane. 



The fibrous portion of the synovial membranes is more 

 dense and resisting and less elastic than the serous mem- 

 branes. It is composed of white inelastic fibrous tissue, 

 with a few elastic fibres and blood-vessels. The latter are 

 generally not so numerous as in the serous membranes. 

 The internal surface is lined with small cells of flattened, 

 pavement-epithelium, with rather large, rounded nuclei. 

 These cells exist in from one to two or four layers. 1 



In most of the joints, especially those of large size, as 

 the knee and hip, the synovial membrane is thrown into 

 folds which contain a considerable amount of true adipose 

 tissue. In nearly all the joints, the membrane presents 

 fringed, vascular processes, called sometimes synovial fringes. 

 These are composed of looped vessels of considerable size ; 

 and when injected they bear a certain resemblance to the 

 choroid plexus. The edges of these fringes present numer- 

 ous leaf-like, -membranous appendages, of a great variety of 

 curious forms. They are generally situated near the attach- 

 ment of the membrane to the cartilage. There is no reason 

 for supposing that either the adipose folds or the vascular 

 fringes have any special office in the production of the 

 eynovial secretion, different from that of other portions of 

 the membrane, though such a theory has been advanced. 



1 KOLLIKER, Handbuch der Gewebelehre des Menschen, Leipzig. 1867, S. 201. 



