210 THE ORGANS 



DiARTiiROSis. — In diarthrosis must be considered (a) the artic- 

 ular cartilages, (b) the glenoid ligaments and interarticular cartilages, 

 (c) the joint capsule. 



(a) Articular cartilages cover the ends of the bones. They are 

 of the hyaline variety,^ being the remains of the original cartilag- 

 inous matrix in which the bones are formed. Next to the bone is a 

 narrow strip of cartilage in which the matrix is calcified. This is 

 separated from the remaining uncalcified portion of the cartilage by 

 a narrow so-called ''striated" zone. The most superficial of the 

 cartilage cells are arranged in rows parallel to the surface; in the mid- 

 region the grouping of cells is largely in twos and fours as in ordinary 

 hyaline cartilage (page 98) ; while in the deepest zone of the uncal- 

 cified cartilage the cells are arranged in rows perpendicular to the 

 surface. 



(b) The glenoid ligaments and interarticular cartilages conform 

 more to the structure of dense fibrous tissue than to that of cartilage. 



(c) The joint capsule consists of two layers, an outer layer of 

 dense fibrous tissue intimately blended with the Hgamentous struc- 

 tures of the joint and known as the stratum Jibrosum, and an inner 

 layer, the stratum synoviale or synovial membrane, which forms the 

 lining of the joint cavity. The outer part of the stratum synoviale 

 consists of areolar tissue with its loosely arranged white and elastic 

 fibres interlacing in all directions, and scattered connective- tissue cells 

 and fat cells. Nearer the free surface of the membrane the fibres 

 run parallel to the surface and the cellular elements are more abun- 

 dant. The cells are scattered among the fibres and are stellate branch- 

 ing cells like those usually found in fibrous connective tissue. On 

 the free surface, however, the cells lie close together, in places form- 

 ing a single surface layer, in other places being disposed in three 

 or four layers. Formerly described as endothehum, they are 

 now generally considered connective- tissue cehs or "mesenchymal 

 epithelium." 



From the free surfaces of synovial membranes, processes {syno- 

 vial villi — Haversian fringes) project into the joint cavity. Some of 

 these are non-vascular and consist mainly of stellate cells similar to 

 those of the synovial membrane. Others have a distinct core of 

 fibrous tissue containing blood-vessels and covered ^\ith stellate con- 



^In the acromio-clavicular, sterno-clav'icular, costo-vertebral, and maxillary articu- 

 lations the cartilage is of the fibrous form. The same is true of the cartilage covering 

 the head of the ulna, while the surface of the radius, which enters into the wrist-joint, 

 is covered not by cartilage, but by dense fibrous tissue. 



