CHAP. VI.] DIABTHBODIAL JOINTS. 135 



articulation between the bodies of the vertebrae, that between the 

 two ossa pubis at what is called the symphysis, and that between 

 the ilium and sacrum. Like the sutures, the amphiarthroses are 

 liable to become obliterated in old age, by the ossification of the 

 interosseous lamina. This is not common in the interpubic, and 

 occurs now and then in the inter vertebral and sacro-iliac joints. 



Diarthrosis. Mobility is the distinguishing characteristic of this 

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

 by a lamina of cartilage (diarthrodial cartilage), having a synovial 

 sac interposed, and in some cases two, separated by a meniscus. 

 The integrity of the articulation is maintained by ligaments which 

 pass from one bone to the other. Their mechanism is much more 

 complicated than that of synarthrodial joints, being intended not 

 only for security, but also to give a certain direction to the motions 

 of which they are the centre. 



Before proceeding to the enumeration of the varieties of joints 

 that come under this head, it will not be amiss to describe briefly 

 the various motions which may take place between any two seg- 

 ments of a limb, and which it is the object of these joints to admit 

 of. It is obvious, that the most simple kind of motion which can 

 exist between two plane or contiguous surfaces, is that of gliding : 

 one surface glides over the other, limited by the ligaments which 

 extend between the bones. This motion, however, is not confined 

 to plane surfaces ; it may exist evidently between contiguous sur- 

 faces, whatever their form. When two segments of a limb, placed 

 in a direct line, or nearly so, can be brought to form an angle with 

 each other, the motion is that of flexion, the restoration to the direct 

 line is extension. These two motions belong to what Bichat calls 

 limited opposition ; the flexion and extension of the fore-arm on the 

 arm illustrate it. Sometimes a motion of this kind takes place in 

 four directions, indicated by two lines which cut at right angles. 

 This is best understood by a reference to the motions which take 

 place at the hip-joint : there it will be seen that the thigh-bone may 

 be brought forward so as to form an angle with the trunk, flexion 

 or it may be restored, extension ; it may be separated from the 

 middle line of the body so as to form an angle with the lateral sur- 

 face of the trunk, abduction or it may be restored and made to 

 approximate the middle line, adduction. It is evident that a joint, 

 which is susceptible of these four motions, may also move in the in- 

 termediate directions. When these motions are performed rapidly, 

 one after the other, one continuous motion appears, in which the 

 distal extremity of the bone describes a circle indicating the base of 



