140 



ARTICULATIONS OF THE UPPER LIMB. 



the radius and ulna. Its superior border is placed about an inch below 

 the tubercle of the radius, leaving a space (hiatus interosseus) through 

 which the posterior iuterosseous vessels pass. This space is diminished in 

 size by the round or oblique ligament, a thin, narrow fasciculus of fibres ex- 

 tending obliquely downwards and outwards from the coronoid process, to be 

 attached to the radius about half an inch below the tubercle. Other small 

 bundles of fibres, having the same direction as the round ligament, are often 

 to be found at intervals, decussating with the fibres of the interosseous 

 ligament on its posterior surface. 



MOVEMENT OP THE RADIUS ON THE ULNA. The disposition of the annular liga- 

 ment allows the head of the radius to rotate freely within it, while the lower end of 

 the radius, bound by the triangular fibro-cartilage to the styloid process of the ulna, 

 has a freedom of circumduction round that point, by which the hand is brought into 

 the prone or the supine position. Thus in pronation and supination the movement 

 of the radius describes a part of a cone, the axis of which extends from the centre of 

 the head of the radius to the styloid process of the ulna. 



THE ELBOW- JOINT. The lower extremity of the humerus is in contact 

 with the ulna and radius at the elbow, and forms with them a hinge-joint. 

 The greater sigmoid cavity of the ulna articulates with the trochlea of the 

 humerus, so as to admit of flexion and extension only ; while the cup- 

 shaped depression on the head of the radius is fitted to turn freely on the 

 rounded capitulum. These bones are united principally by lateral ligaments. 



Fig. 130. LIGAMENTS OP THE ELBOW-JOINT. J 



A, from the outer side and behind ; B, from the front ; C, from the inner side and 

 behind. 1, internal lateral ligament ; 2, external lateral ; 3, the middle strongest part 

 of the anterior ligament ; 4, orbicular ligament ; 5, posterior, represented as wrinkled 

 from relaxation in extension. In these figures the round ligament and upper part of the 

 interosseous membrane are also represented below the elbow -joiut. 



