ARTHROLOGY 



Kinds of Joints. — Joints may be movable, immovable, or mixed. 

 In movable, or true joints the articular surface of each bone is covered 

 by cartilage. The bones are held together by ligaments, the 

 capsular one often surrounding the joint and enclosing the sjmovial 

 membrane. In some joints there is a pad of fibrous cartilage 

 interposed between the two articular cartilages. Such a pad, 

 called a meniscus, adds to the elasticity and the free movement 

 of the joint. Movable joints form the most numerous class; they 

 are for the most part found in the limbs. 



In an immovable joint there is only a thin layer of fibrous or 

 cartilaginous material interposed between the bones. The fibrous 

 layer of the periosteum of both bones unite to cover the connecting 

 material and becomes attached to the same, thus serving as a liga- 

 ment. If the connecting material is fibrous, the joint is called a 

 suture; if cartilaginous, a synchondrosis. These joints are found 

 in the skull and in the pelvis. 



The term mixed is used with reference, not to the motion in joints, 

 but to their structure, which partakes of the nature of both the 

 movable and immovable. The bones are firmly joined by a strong 

 interposed pad of fibrous cartilage to which also is adherent the 

 ligaments of the joint. There are no capsular ligaments; the 

 cartilaginous pad or disc is softer toward its center, where occasion- 

 ally there may be one, or even two, narrow cavities. Authorities 

 differ as to whether such cavities are lined by synovial membrane 

 or not. Since there are really no frictional surfaces in such a joint, 

 motion depends upon the flexibility of the disc. The joints between 

 the vertebral centra afford the best illustration of the mixed class. 



Movement of Joints. — The movements admissible in joints may 

 be divided into four kinds: gliding, angular movement, circumduc- 

 tion, and rotation. These movements are often, however, more or 

 less combined in the variour joints. It is seldom that there occurs 

 only one kind of motion in any particular joint. 

 Gliding movement is the most simple kind of motion that can 



56 



