NORMAL HISTOLOGY. 



Where two joint-cavities exist, separated by an interarticular cartilage, 

 the development of the partition, or meniscus, seems to influence the struct- 

 ure of the cartilage capping the articular surfaces of the bones. In such 

 cases, as in the mandibular, costo-sternal, sterno-clavicular, acromio-clavic- 

 ular and lower radio-ulnar articulations, fibrous cartilage not only forms 

 the interarticular plates, but also contributes the covering of the bones. 

 Such instances, therefore, are exceptions to the usual investment of hyaline 

 cartilage. The adaptation cartilages, or labra glenoidalia, as the glenoid 

 and semilunar cartilages in the shoulder and knee joint respectively, that 

 serve to deepen the cups in which the humerus and the femur play, also 

 consist of dense fibrous tissue containing rounded cartilage-cells. 



The capsule surrounding the joint-cavity includes two layers : the 

 outer fibrous and the inner synovial. The fibrous layer, made up of inter- 

 lacing bundles of dense fibrous tissue, varies much in thickness in different 



Free surface of 

 articular cartilage 



Bom 



Marrow-tissue 



,---'', ^ *""---->.- >:. 



Blood-vessel 



f^f-t 'Synovial membrane 



Junction of 



^"^ cartilage and 

 , v^'- synovial membrane 



FIG. 64. Section through margin of joint, showing articular cartilage and synovial membrane. X 100. 



joints, in the minute articulations between the ear-ossicles being a delicate 

 membrane, while in the capsule of the hip-joint it reaches almost a centimeter. 



The synovial layer, or synovial membrane, consists of loose connective 

 tissue containing elastic fibres and more or less extensive groups of fat- 

 cells; next the joint-cavity the tissue is condensed into a narrow compact 

 stratum, whose free or joint-surface is clothed with flattened connective 

 tissue cells. The latter are plate-like elements, irregularly oval or stellate in 

 outline, and, where closely placed, form a lining for the capsule that resembles 

 an imperfect endothelium. While the fibrous layer of the capsule is often 

 carried for some distance beyond the margin of the joint to blend with the 

 periosteum, the synovial membrane is reflected from the capsule to the 

 bones and the articular cartilage, extending over the latter for a variable 

 distance, but thinning out and disappearing over the surfaces subject to 

 pressure. The actual articulating surfaces, therefore, are devoid of synovial 

 membrane, the lubricated cartilages coming into contact during the move- 

 ments of the bones. 



Within the larger- articulations the synovial membrane is thrown into 

 uncertain folds, which project into the joint, enclose masses of adipose tissue, 

 and are beset with numerous minute elevations. The latter, the synovial 

 villi, are found especially around the margin of the articular surfaces and, 



