CHAP. V.] 



JOINTS. 



49 



broad, flattened disks of fibro-cartilage, as in the articulations 

 between the bodies of the vertebrae. These inter vertebral 

 disks being compressible and extensile, the spine can be moved 

 to a limited extent in every direction. In the pelvis the articu- 

 lation between the two pubic bones (symphysis pubis), and 

 between the sacrum and ilia (sacro-iliac articulation), are also 

 slightly movable. The pubic bones are united by a disk of 

 fibro-cartilage and by ligaments. In the sacro-iliac articulation 

 the sacrum is united more closely to the ilia, the articular sur- 

 faces being covered by cartilage and held together by ligaments. 

 The movable articulations. This division includes the com- 

 plete joints, joints having a secreting membrane placed be- 

 tween their opposing surfaces, which keeps them well lubricated 

 and capable of free movement one upon the other. Each articular 

 end of the bone is covered by cartilage, which provides surfaces 



of remarkable smoothness, and 

 these surfaces are lubricated by 

 the synovial fluid secreted from 

 the delicate synovial membrane 

 which lines the cavity of the 

 joint. This membrane is contin- 

 uous with the margin of the ar- 

 ticular cartilage, and along with 

 them completely encloses the 

 joint cavity. The bones are 

 united by fibrous connective 

 tissue in the various forms of 

 ligaments, such as membranous 

 capsules, flat bands, or rounded cords. These ligaments are not 

 always so tight as to maintain the bones in close contact to all 

 positions of the joint, but are rather tightened in some positions 

 and relaxed in others, so that in many cases they are to be looked 

 upon chiefly as controllers of movements, and not as serving 

 solely to hold the bones together. The bones are mainly held 

 together in these joints by atmospheric pressure and by the 

 surrounding muscles. 



The varieties of joints in this class have been determined 

 by the kind of motion permitted in each. They are as 

 follows : 



(1) Gliding joint. The articular surfaces are nearly flat, 



SYNOVIAL FOLD 



FIG. 50. A SIMPLE COMPLETE JOINT. 

 The synovial membrane is represented 

 by dotted lines. 



