THE JOINTS 



The study of the articulation of the bones and the ligaments 

 holding them in place is known as syndesmology. There are 

 three general classes of articulations: Synarthroses, or im- 

 movable joints; amphiarthroses, or joints with slight motion; 

 and diathroses, or joints freely movable. A synarthrodal 

 joint is exemplified in the sutures between the bones of the 

 skull. The two surfaces of bone are 

 separated by fibrous membrane only. 

 An amphiarthrodal articulation oc- 

 curs between the bodies of the ver- 

 tebrae and also in the union of the 

 sacrum with the ilium. In the ar- 

 ticulation of the vertebrae the con- 

 tiguous surfaces of the bone are 

 faced by flattened discs of iibro- 

 cartilage (Fig. 22). 



The diarthrodal articulation is ex- 

 emplified in all joints allowing free 

 motion. Such joints are composed 

 of the following parts: the ends 

 of two bones having their contiguous surfaces covered 

 with cartilage; a synovial membrane forming a short tube 

 whose ends are closed by the cartilaginous articulating 

 surfaces; and several ligaments varying in number from two 

 or three to a dozen (Fig. 45). There are four kinds of the diar- 

 throsis: the arthrodia, or gliding joint, which occurs between 

 the bones of the carpus; the enarthrosis, or ball-and-socket 

 joint, such as at the hip or shoulder; the ginglymus, or hinge 

 joint, exemplified at the elbow and knee; and the rotatoria, 

 or pivot joint, formed by the articulation of the axis and atlas. 



As the knee-joint can be readily dissected and the parts 

 well displayed; it is here described somewhat in detail. The 



83 



Fig. 45. — Diagram of a 

 Diarthrodal Joint. 

 ar, Articulatory cartilage; 

 Ig, ligament; sn, synovial mem- 

 brane; sy, synovial fluid in 

 synovial cavity; /, tibia; v, 

 femur. 



