ARTICULATIONS 153 



facets of the os naviculare and os lunatum, and a very small 

 articular facet on the extreme lateral part of the proximal 

 surface of the os triquetrum. Two interosseous ligaments 

 stretch across the narrow intervals between these bones 

 one on either side of the os lunatum and complete the 

 carpal surface. Formed of these factors the carpal surface 

 is convex in all directions. Further, it should be observed 

 that the articular surface extends distally to a greater extent 

 dorsally than on the volar aspect. 



The proximal surface or socket (Fig. 61) is elongated from 

 side to side, and concave in all directions. The greater part 

 of it is formed by the distal end of the radius, but to the 

 medial side of this by the discus articularis of the distal radio- 

 Head of ulna 



Styloid process 

 , of ulna 



Surface for os,. 



naviculare man us m*-^-. .< - // fj . 



Apex of discus 



^ , articularis 



Groove for tendon of ext. 

 pollicis longus 



FIG. 61. 1 Carpal Articular Surfaces of the Radius and of the Discus 

 Articularis of the Wrist. 



ulnar joint likewise enters into its construction. The distal 

 articular surface of the radius is divided by a low ridge into 

 a lateral triangular and a medial quadrilateral facet. The 

 lateral facet, in the ordinary position of the hand, is in 

 contact with the greater extent of the proximal articular 

 surface of the os naviculare. The medial facet of the radius, 

 together with the discus articularis, forms a much larger 

 surface, triangular in outline, which is opposed to the 

 proximal articular surface of the os lunatum. When the 

 hand is placed in line with the forearm no part of the 

 proximal articular surface is allotted to the os triquetrum : 

 its small articular facet rests against the medial part of the 

 capsule of the joint. When the hand is moved medially 

 (i.e. adducted), however, the os triquetrum travels laterally, 

 and its articular surface comes into contact with the distal 



