SYNOVIAL MEMBRANES. 315 



1. The corium (or layer of areolar tissue) sometimes, though 

 rarely, contains fat-cells in its meshes, and a few scattered cartilage- 

 cells with thick, opaque walls. 



2. The epithelium is composed of from one to four layers of large 

 tessellated cells, 3 J UTJ to y^ of an inch in diameter, with roundish 

 nuclei of g^ to ^^ of an inch. 



The synovial membranes present large adipose masses and vascu- 

 lar processes. The. former, once erroneously termed Haversian 

 glands, are found principally in the hip- and knee-joints, and consist 

 of collections of fat-cells in vascular portions of the synovial mem- 

 brane. The vascular processes constitute red, flattened projections 

 of the synovial membrane, with an indented and polished margin, 

 furnished with minute processes. The folds are usually placed 

 close to the junction of the synovial membrane with the articular 

 cartilage, and lie flat upon the latter, forming a sort of coronal 

 around it. They differ from the rest of the membrane in structure, 

 principally in their great vascularity, since they consist of little 

 else than minute arteries and veins, and delicate capillaries forming 

 wavy loops at the edge of the processes ; and hence resemble the 

 choroid plexus in the ventricles of the brain. Besides the vessels, 

 these processes consist of areolar tissue and the epithelium found 

 elsewhere on these membranes. At the edge of these processes, 

 projections of the membrane, of extraordinary forms (sometimes 

 resembling the stems of a cactus), are found. It is these non- 

 vascular processes which, being enlarged and then detached, con- 

 stitute one of the forms of the erroneously so-called loose cartilages 

 in joints, as has already been shown (p. 319, 2). 



The nerves of the synovial membrane are but few in number. 



The synovia is evidently secreted by the epithelial cells upon the 

 vascular processes ; and to a very slight extent also, doubtless, by 

 those on the rest of the membrane. The properties of this fluid 

 have already been stated (pp. 180 and 198). Its function is to 

 diminish friction in the varied movements of the joints. 



Inter -Articular Fibro- Cartilages. 



The inter-articular fibre-cartilages may be mentioned here, though 

 they do not actually come into contact with the bones. They are 

 interposed between the two articular-cartilages of some joints (arti- 

 culations of lower jaw, sternum and clavicle, &c.); or form mere 

 projections between them (semilunar cartilages, so called, of the 



