32 



BONES, JOINTS AND LIGAMENTS 



Characteristics of a true joint. The three charac- 

 teristics of a true joint are the smooth cartilage-covered 

 ends of the bones ; the synovial membrane ; and the liga- 

 ments, which completely enclose the joint and, while 



Frontal bone 



Suture 



Occipital bone 



FIG. 25. The skull viewed from above, showing arrangement of bones and method 

 of locking together. 



permitting natural movement, are tough and tight 

 enough to prevent any movement harmful either in 

 direction or extent. 



Kinds of joints. Joints are classified according to 

 their range and character of movement, which depend 

 upon their shape and the arrangement and tightness 

 of the ligaments. The joints permitting the greatest 

 range are the ball-and-socket joints, in which the 

 rounded head of one bone moves in the hollow socket of 

 another. Their capsular ligaments are sufficiently loose 

 to allow the ball to move freely in its socket up to the 



