RADIO-ULXAll ARTICULATIONS. 



151 



cartilage is a thick plate attached by its base to a ridge separating the 

 carpal liom the ulnar articulating surface of the radius ; and by its 



pj„ ige, — The upper part op the Ulna, with the Fig. 130. 



Orbicular Ligament op the Radius, h 



1 upper division of the sigmoid surface on the olecranon ; 

 2 extremity of the coronoid process ; 5, orbicular ligament. 



apex to a depression at the root of the styloid 

 process of the ulna, and to the side of that pro- 

 cess. Its upper surface looks towards the ulna, 

 its lower towards the cuneiform bone, and it 

 separates the inferior radio-ulnar articulation 

 from the wi'ist-joint. The synovial mem- 

 brane, sometimes called from its looseness )iic))i- 

 Irana saccifonnis, extends U})wards between the 

 radius and ulna, and horizontally inwards between 

 the ulna and triangular fibro-cartilage. When 

 the fibro-cartilage is perforated, as is occasionally 

 tlie case, this synovial cavity communicates with 

 that of the wrist-joint. 



The interosseous membrane or ligament of the forearm is a thin, 



Fig. 137. 



Fig. 137. — The lower parts op the Radius and 

 Ulna, with the Triangular Fibro-Cartilage 

 connecting them. § 



1, ulna ; 2, its styloid process ; 3, radius ; 4, 

 articular surface for the scaplioid bone ; 5, that for 

 the semilunar boae ; 6, lower surface of the tri- 

 p.ngular fibro-cartilage ; * *, a piece of whalebone 

 passed between the fibro-cartilage and the ulna. 



flat, fibrous membrane, the direction of 

 whose fibres is for the most part ob- 

 liquely downwards and inwards, from 

 the interosseous ridge of the radius to 

 that of the ulna. Its superior border is 

 placed about an inch below the tubercle 



of the radius, leaving an open space above (hiatus interosseus) 

 through which the posterior interosseous vessels pass. This space is 

 diminished in size by the round or oblique ligament, a thin, narrow 

 fasciculus of fibres extending 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 Of the Radius on the Ulna. — The disposition of the annular 

 ligament 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 radius describes a part of a cone, the axis of which 



