LECTURE A". 

 ARTICULATIONS OR JOINTS. 



Articulations are divided into several groups for the purpose 

 of study. These groups are : immovable, slightly movable, and 

 freely movable. 



Examples: Immovable, between the skull bones; slightly 

 movable, between vertebrae; freely movable, between scapula and 

 humerus. 



Freely movable.— The freely movable articulations are subdi- 

 vided according to shapes of the articular surfaces and varieties of 

 movement that can be produced. These subdivisions are : ball and 

 socket, hinge, pivot, imperfect hinge, and gliding. 



The ball and socket articulation, of which we find examples 

 at the shoulder and hip, is made by a rounded head of one bone 

 fitted into a rounded cavity of some other bone ; for instance, at 

 the shoulder we have a rounded head of the humerus fitted into 

 a glenoid cavity of the scapula. At the hip a rounded head on the 

 superior extremity of the femur fits perfectly into the cotyloid 

 cavity of the pelvic bone. It will be readily seen that the ball 

 and socket joint permits the greatest variety of movements. 



The hinge joint is made by two articular surfaces of such 

 shape and so fitted together that no lateral or rotarv motion is 

 possible. Only two movements are allowed at joints of this kind: 

 flexion and extension. The articulation between the humerus 

 above and the ulna and radius below offers a good example of 

 this kind. 



Pivot joint is one where portions of two bones are in con- 

 tact in such a wav as to permit of rotary motion ; for example, 

 between the Atlas and axis, the rotation being around the odon- 

 toid or tooth-like processes of the axis. Articulations of this kind 

 permit of rotation only. 



The imperfect hinge joint is one which permits of two 

 -principle motions — flexion and extension, and to a limited extent 



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