OP THE ARTICULATIONS. 399 



arthroses also sometimes form in the place of some diarthro- 

 ses, of which the synovial membrane has contracted flexible 

 adhesions. 



627. Diarthrosis is a kind of articulation in which the 

 articular surfaces of the bones are in contact, and move upon 

 each other. 



This kind of articulation exists among all the bones of the 

 limbs, whether between each other, or between them and the 

 trunk, between the lower jaw and the skull, between the skull 

 and vertebral column, between the articular processes of the 

 vertebras, between the ribs and vertebrae, and between the 

 costal cartilages and the sternum. 



628. The articular parts of the bones, in this kind of arti- 

 culation, are broad surfaces, whose configuration is reciprocal. 

 These surfaces are in general, the one convex, the other con- 

 cave. The convex surfaces, or articular eminences, are some- 

 times rounded like a large segment of a sphere, in which case 

 they are called heads. Others are rounded, but elongated in 

 one direction, and contracted in another: these have been 

 named condyles. The heads and condyles are sometimes sup- 

 ported by a narrow part, which is called the neck. The arti- 

 cular depressions, or concave surfaces, bear the name of 

 cotyloid cavities, when they have the form of a segment of a 

 sphere and are deep; and that of glenoid cavities, when they 

 are more superficial. Sometimes two condyles are brought 

 near each other laterally, and leave between them a neck 

 which enters into the articulation like themselves. This kind 

 of surface is named a pulley, trochlea: Lastly, many articular 

 surfaces, which are nearly flat, presenting little convexity or 

 concavity in their configuration, have received no particular 

 name, but are designated, according to their extent, under the 

 generic names of articular surfaces or facettes. 



All these surfaces are covered with diarthrodial cartilages 

 (554). These cartilages are themselves covered by synovial 

 membranes (210), and moistened with synovia (216). There 

 are, moreover, between certain of these surfaces, menisci or 

 chondroid inter-articular cartilaginous ligaments (531.) 



629. The means of union are fibrous ligaments, (512.) 



52 



