iv JOINTS 57 



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 con- 

 structed on the same general plan, but, owing partly to 

 the form of the adjacent surfaces, partly to the mode 

 of attachment 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. Strong ligaments connect the neural arches 

 with one another and join 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 inter- 

 posed between their adjacent cartilage-covered surfaces. 

 Where little or no movement 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 immovable 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 free movement between the bones. 



