12 ANTHROPOLOGY. 



are two kinds, squamosa, when the bevelled edge of one overlaps and rests 

 on the other, as in the temporo-parietal suture ; and harmonia, where there 

 is simple apposition, as seen in the union of most bones of the face. 



h. Schindylesis. This second form of synarthrosis is where a thin plate 

 of bone is received into a space or cleft formed by the separation of two 

 laminae of another bone, as seen in the insertion of the azj^gos process of 

 the sphenoid bone into the fissure on the superior margin of the vomer. 



c. Gomphosis. Here one bone is inserted into a cavity in another, just 

 as a nail is driven into a board, or a tree implanted by its roots in the 

 ground. The sole instance in the human subject is seen in the insertion of 

 the teeth in the alveoli or sockets. 



d. Amphiarthrosis. This is an articulation where two plane or mutually 

 adapted surfaces are held together by a cartilaginous or fibro-cartilaginous 

 lamina of considerable thickness, as well as by external ligaments. By 

 reason of the elasticity of the interposed lamina, the joint possesses a mani- 

 fest though slight degree of motion. Examples of this form of joint are 

 found in the articulations between the bodies of the vertebae, that between 

 the two bones of the pubes. and that between the ilium and sacrum. 



B. DiARTHROSlS. Evident mobility is the distinguishing characteristic 

 of this class of joints ; the articular surfaces are contiguous, each covered 

 by a lamina of cartilage (diarthrodal cartilage) having either one or two 

 synovial sacs interposed. The integrity of the articulation is maintained 

 by ligaments which pass from one bone to the other. The varieties are as 

 follows : 



a. Artlirodia. Here the surfaces are plane, or nearly so: the motion is 

 that of gliding, limited in extent and direction only by the ligament of the 

 joint or by processes of the bones. Examples are seen in the articular 

 processes of the vertebrae, in the radial, the carpal, the sterno-clavicular, 

 and other joints. 



h. Enarthrosis. This, sometimes termed a hall and socket joint, is where 

 the globular head of one bone plays in a cup-like cavity of another, or 

 others. The ball is kept in place by a capsular ligament. Sometimes there 

 passes a straight ligament from the head of the ball to be inserted into the 

 bottom of the socket. An instance of this is seen in the attachment of the 

 thigh bone to the pelvis. The shoulder aifords a second instance of the 

 ball and socket joint. 



c. Gingli/mus or hinge joint. Here the articular surfaces are marked 

 with elevations and depressions, which exactly fit into each other, so as to 

 restrict motion in all but one line of direction. They are always provided 

 with strong lateral ligaments, which are the chief bonds of union of the 

 articular surfaces. Perfect examples of this articulation are furnished by 

 the elbow and ankle joints. The knee also, and the phalangeal joints, are 

 true ginglymi. 



d. Trecharthrosis. A pivot and a ring constitute the mechanism of this 

 form of joint. The* ring is generally formed partly of bone and partly of 

 ligament; it sometimes moves on the pivot, and sometimes the pivot moves 

 in it. The motion is confined to rotation, the axis of which is the axis of 



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