302 



THE AETICULATIONS OR JOINTS. 



bones forming such a joint. Ligaments have therefore additional importance in 

 this group, for not only do they constitute the uniting media which bind the 

 articulating bones together, but, to a large extent, they form the peripheral 

 boundary of the joint cavity, although not equally developed in all positions. 

 Thus, every diarthrodial joint possesses a fibrous or ligamentous envelope con- 

 stituting the fibrous stratum of the articular capsule, which is attached to the ad- 

 jacent ends of the articulating bones. For special purposes, particular parts of the 

 fibrous stratum may undergo enlargement and thickening, and so constitute strong 

 ligamentous bands, although still forming continuous constituents of the envelope. 

 The fibrous stratum is lined by a stratum synoviale (O.T. synovial membrane), 

 the two strata constituting the capsula articularis. The synovial stratum is con- 

 tinued from the inner surface of the fibrous stratum to the surface of the intra- 

 articular portion of each articulating bone. The part of the bone included within 

 the joint consists of a " non-articular " portion covered by the synovial layer and 

 an " articular " portion covered by encrusting hyaline cartilage. The latter provides 

 the surface which comes into apposition with the corresponding area of another 

 bone. In its general disposition the synovial layer may be likened to a cylindrical 

 tube open at each end. This layer is richly supplied by a close network of 

 vessels and nerves. 



Certain diarthroses present intracapsular structures which may be distinguished 

 as interarticular ligaments and articular discs and menisci (O.T. interarticular 

 fibro-cartilages). 



Ligamenta Interartidularia. Interarticular ligaments extend between, and are 

 attached to, non-articular areas of the intracapsular portions of the articulating 

 bones. They usually occupy the long axis of the joint, and occasionally they 

 widen sufficiently to form partitions which divide the joint-cavity into two com- 

 partments, e.g. the articulation of the heads of the ribs with the vertebral column, 

 and certain of the costo-sternal joints. 



Articular discs and menisci (O.T. interarticular fibro-cartilages) (Fig. 291) are 

 more or less complete partitions situated between and separating opposing articular 

 surfaces, and when complete they divide the joint cavity into two distinct 



compartments. By its periphery, a disc is 

 rather to be associated with the articular 

 capsule than with the articulating bones, 

 although its attachments may extend to 

 non - articular areas on the latter. Those 

 found in the knee-joint are called menisci ; 

 those found in other joints are called articular 

 discs. 



Both interarticular ligaments and articu- 

 lar discs and menisci have their free surfaces 

 covered by the synovial stratum. 



Adipose tissue, forming pads of varying size, 

 is usually found in certain localities within 

 the joint, between the synovial stratum and 

 the surfaces which it covers. These pads are 

 FIG. 291. DIAGRAM OF A DIARTHRODIAL JOINT soft and pliable, and act as packing material, 

 WITH ARTICULAR DISC DIVIDING THE JOINT- filling up gaps or intervals in the joint. 



CAVITY INTO TWO COMPARTMENTS. T , . & , J 



During movement they adapt themselves to 



the changing conditions of the articulation. 



In addition to merely binding together two or more articulating bones, ligaments 

 perform very important functions in connexion with the different movements 

 taking place at a joint. They do not appreciably lengthen under strains, and thus 

 ligaments may act as inhibitory structures, and by becoming tense may restrain 

 or check movement in certain directions. 



Synovial strata, in the form of closed sacs termed mucous or synovial bursse, are frequently 

 found in other situations besides the interior of joints. Such bursae are developed for the 

 purpose of reducing the friction, (a) between the integument and certain prominent subcutaneous 

 bony projections, as, for instance, the point of the elbow, or the anterior surface of the patella 



Cartilage 

 articularis 



