DISLOCATIONS. 



65 



hand on it near the wrist, and then turn the hand over ; 

 the lower end of the radius will be -found to cross over the 

 ulna and to be on its inner side (Fig. 27, B), when tho 

 movement is completed ; in this 

 position the hand is said to be 

 in pronation. 



The lower end of the hume- 

 rus (Fig. 21) has a large artic- 

 ular surface ; on the inner two- 

 thirds of this, Ir, the ulna fits, 

 and the grooves and ridges of 

 the bones interlocking form a 

 hinge -joint, allowing us only to 

 bend or straighten the elbow- 

 joint. The radius fits on the 

 rounded outer third, Cpl, and 

 rotates there when the hand 

 is turned over, the ulna form- 

 ing a fixed bar around which it 

 moves. B A 



n-lirKno- imnfc no a vnln i^ov FlG 27- A, arm in Bupination ; 



Witting joints as a rule pei- B arm In pronat i n; a;fiumenw; 

 mit of but little movement. jR ' radius; ^ ulna ' 

 Examples are found between the closely-packed bones of 

 the carpus and tarsus (Fig. 19), which slide a little over 

 one another when subjected to pressure. 



Dislocations. When a bone is displaced at a joint or 

 dislocated, the ligaments are more or less torn and other 



What movement is allowed between ulna and humerus? What 

 between radius and huraerus? Around what does the radius rotate 

 when we turn the hand over? 



Do gliding joints allow free movement? Give instances of gliding 

 joints. 



What is a dislocation? What parts are injured when a joint is 

 dislocated? 



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