AMPfllARTHROSIS. 



127 



DIARTHHOSIS. 



In movable or true joints the articular surface of each bone is 

 covered by cartilage of encrustation ; the bones are held together 

 "by ligaments, the capsular one closing the cavity of the joint, 

 fvhich is lined by synovial membrane. In some joints there is 

 ^ pad of fibro-cartilage interposed between the two articular 

 'Cartilages. Such a pad is called a meniscus, 

 and it adds to the elasticity and freedom of the 

 joint. Movable joluts form the most numerous 

 ilass; they are found in the limbs and else- 

 T^^here. The chief varieties are the arthrodia, 

 Knarthrosis, and the ginglymus. 



~ In Arthvodia the motion is slight and glid- 

 ing, the bony surfaces being flat or slightly 

 undulating, and the motion limited by ligaments 

 or processes of bone, as in the small bones of 

 the carpus and tarsus. Enarthrosis, the ball- 

 -and-socket joint, is capable of moving in any 

 <iirection ; the extremity of one bone is globular, 

 and fits into a cup-shaped concavity in another 

 bone ; the hip and shoulder joints are examples. 

 The Ginglymus, or hinge joint, although it may 

 allow extensive motion, limits it to one plane, 

 backwards and forwards, as in the elbow. A 

 Rotatory joint (diarthrosis rotatorius), where the 

 motion is limited to rotation, is formed by a pivot on one bone 

 fitting into a ring on another, as the atlo-axoid joint. 



Fig. 45. 



Diagram of a diar- 

 throdial joint, a a, 

 Articulating bones ; 

 c c c, Synovial mem- 

 brane. The sliaded 

 portions represent 

 the articular carti- 

 lages, which are said 

 by some to be covered 

 in early foetal life by 

 the inflected con- 

 tinuations (b h) of 

 the synovial mem- 

 brane. 



AMPHIARTHROSIS. 



There is but one kind of amphiarthrosis or mixed joint ; the 

 variations are too slight to warrant separation into classes. 



The term mixed is used with reference, not to the motion 

 in such joints, but to their structure, which partakes of the 

 nature of both the movable and immovable, the bones being 

 £rmly joined together by a strong interposed pad of fibro- 

 cartilage, which is likewise adherent to the ligaments of the 

 joint. There are no capsular ligaments; the cartilaginous 

 pad or disc is softer towards the centre, where occasionally 



