IV . JOINTS 57 



joint, and is capable of movement in any plane. A similar 

 but less perfect cup-and-ball joint is that of the shoulder, 

 in which the cup is furnished by the glenoid cavity, the ball 

 by the head of the humerus. 



Other Joints, — The elbow- and knee-joints are constructed 

 on the same general plan, but, owing partly to the form of 

 the adjacent surfaces, partly to the mode of attach- 

 ment of the ligaments, they are capable of movement 

 in one plane only, i.e., up and down, but not from side to 

 side. They are therefore distinguished as hinge-joints. 



The vertebrae are connected with one another in a similar 

 way. Between the convex hinder face of one centrum and 

 the concave front face of its successor is a synovial capsule, 

 and the two centra are bound together by ligament, a 

 shallow cup-and-ball joint, with a very limited range of 

 movement, being produced. There are also synovial 

 capsules between the articular processes, which, being in 

 contact with one another by flat surfaces and working mainly 

 from side to side, form gliding-joints. There are also strong 

 ligaments connecting the neural arches with one another and 

 joining the first vertebra to the skull. 



In all cases where free movement is necessary the 

 joints are formed in the same way ; the bones are bound 

 together by ligaments, and a synovial capsule is interposed 

 between their adjacent surfaces. When little or no move- 

 ment is required, as between the bones of the shoulder- and 

 hip-girdles, the union is effected by cartilage or ligament 

 only, and there is no synovial capsule. Such joints are 

 therefore distinguished as immoveable or imperfect joints. 



The Muscles. — We see then that the bones of the skeleton 

 are attached to or articulated with one another by means 

 of ligaments, so arranged, in most cases, as to allow of more 

 or less fr^e movement between the bones. We must now 



