THE UPPEK LIMB IN MAN. 237 



and extent of the movements of his shoulder-joint. In full 

 extension there is a complete rotation of the humerus on its 

 axis when the palm is turned upwards, which is due to 

 the spiral twist of the bundles of the deltoid. On the sur- 

 face of the head of the human humerus is a distinct area 

 devoted to the movement of extension, which corresponds 

 to the extension area of the femur. The larger figure (Fig. 6) 



Fig. 6. 



is the head of the human humerus, the smaller that of the ape. 

 The space lying immediately to the right of the dotted line 

 in the former is the articular area for complete extension 

 which is absent in the ape's humerus. The coracoid area in the 

 glenoid cavity is the extensor area of that aspect of the joint. 

 e. The shoulder-joint in man is alone provided with com- 

 plete articular areas. 



7. — Forearm. 



a. No animal, not even the ape, can either fully extend or 

 fully flex the elbow joint ; nor is any animal capable of com- 

 pleting the movements of pronation and supination. 



b. In the ape, the flexor articular areas are less complete 

 than the extensor, the movement of extension being especially 

 required in swinging from tree to tree ; and the groove be- 

 tween the radial head «»(' the humerus and the trochlea, instead 

 of being nearly direct, as in man, takes the double oblique 

 course of the ulnar trochlea. The articular surface for the 

 radius is confined to the anterior surface of the human 

 humerus, so that it is in the Hexed or semi-ilexed position 



