AMPHIARTHROSIS. 



127 



Gomphosis is the form where one bone is inserted in a cavity 

 or socket in another, as the teeth in the alveoli. 



DIAKTHROSIS. 



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 often surrounding the joint, and 

 enclosing the synovial membrane. In some joints there is 

 a 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 joints form the most numerous 

 class ; they are found in the limbs and else- 

 where. The chief varieties are the arthrodia, 

 enarthrosis, and the gmglyrnus. 



In Arthrud'.a the motion is slight and gliding, 

 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 direction ; the ex- 

 tremity of one bone is globular, and fits into a 

 cup-shaped concavity in another bone ; the hip 

 and shoulder joints are examples. The Gingly- 

 mus, or hinge joint, although it may allow 

 extensive motion, limits it to one plane, back- 

 wards 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 ; 

 -vnovial mem- 

 brane. The shaded 

 portions represent the 

 articular cartilages, 

 which are said by 

 some to be covered in 

 arly tetal life by the 

 inflected continuations 

 (6 6) of the synovial 

 membrane. 



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 

 firmly 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 



