JOINTS 89 



of the fibrous type. These cartilages serve to deepen the sockets in which 

 the respective ends of the femur and humerus move and are known as 

 adaptation cartilages or labra glenoidalia. 



The joint cavity of a diarthrosis is enveloped in a capsule consisting 

 of two layers, an outer fibro-elastic continuous with the periosteum and 

 an inner cellular layer, the synovial membrane, consisting of epithelioid 

 cells forming a mesenchymal epithelium. The function of the synovial 

 membrane is to secrete a lubricating fluid, the synovia, consisting of about 

 94 per cent, water with small amounts of mucoid substances and oil. 

 In the large joints the synovial (serous) membrane is thrown into villus- 

 like folds (Fig. 240). The covering cellular membrane is occasionally 

 imperfect; the cells vary from the flattened, typical mesothelial cells, to 

 the cubic variety (Fig. 44), and rest directly upon a vascular, frequently 

 fatty, fibrous stroma. 



