CLASSIFICATION OF JOIXTS 



285 



Amphiarthroses (slightly movable articulations). In these articulations the 

 contiguous bony surfaces are either connected by broad flattened disks of fibro- 

 cartilage, of a more or less complex struc- 

 ture, as in the articulations between the 

 bodies of the vertebrae; or are united by an 

 interosseous ligament, as in the inferior 

 tibiofibular articulation. The first form is 

 termed a symphysis (Fig. 298), the second 

 a syndesmosis. 



DiarthrOSeS (freely movable articulations). 



This class includes the greater number 



of the joints in the body. In a diarthrodial joint the contiguous bony surfaces 

 are covered with articular cartilage, and connected by ligaments lined by synovial 

 membrane (Fig. 299). The joint may be divided, completely or incompletely, 

 by an articular disk or meniscus, the periphery of which is continuous with 

 the fibrous capsule while its free surfaces are covered by synovial membrane 

 (Fig. 300). 



Ligament 



Disc of 

 fibrocartilage 

 Articular cartilag. 



FIG. 29S. Diagrammatic section of a symphysis. 





Articular cartilage 



Synovial} 



stratum \Articular 

 Fibrous I capsule 

 stratum J 



FIG. 299. Diagrammatic section of a diarthrodial joint. 



Synovial stratum 

 Articular cartilage 

 Articular disk 



'Fibrous stratum 



Fio. 300. Diagrammatic section of a diarthrodial 

 joint, with an articular disk. 



The varieties of joints in this class have been determined by the kind of motion 

 permitted in each. There are two varieties in which the movement is uniaxial, that 

 is to say, all movements take place around one axis. In one form, the ginglymus, 

 this axis is, practically speaking, transverse; in the other, the trochoid or pivot- 

 joint, it is longitudinal. There are two varieties where the movement is biaxial, 

 or around two horizontal axes at right angles to each other, or at any intervening 

 axis between the two. These are the condyloid and the saddle-joint. There is 

 one form where the movement is polyaxial, the enarthrosis or ball-and-socket joint; 



and finally there are the arthrodia or gliding joints. 

 Ginglymus or Hinge-joint. In this form the articular surfaces are moulded 

 to each other in such a manner as to permit motion only in one plane, forward 

 and backward, the extent of motion at the same time being considerable. The 

 direction which the distal bone takes in this motion is seldom in the same plane 

 as that of the axis of the proximal bone; there is usually a certain amount of devia- 

 tion from the straight line during flexion. The articular surfaces are connected 

 together by strong collateral ligaments, which form their chief bond of union. 

 The best examples of ginglymus are the interphalangeal joints and the joint between 

 the humerus and ulna; the knee- and ankle-joints are less typical, as they allow 

 a slight degree of rotation or of side-to-side movement in certain positions of the 

 limb. 



Trochoid or Pivot-joint (articulatio trochoidea; rotary joint) . Where the movement 

 is limited to rotation, the joint is formed by a pivot-like process turning within 



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