164 THE SKELETON. 



other and separated by sharp edges. The superior surface, quadrilateral in form, 

 smooth, and slightly concave, articulates with the scaphoid. The inferior surface 

 articulates with the upper end of the second metacarpal bonfr^ it is convex from 

 side to side, concave from before backward, and subdivided bjfcan elevated ridge 

 into two unequal lateral facets. The posterior or dorsal and anterior or palmar 

 surfaces are rough, for the attachment of ligaments, the former being the larger 

 of the two. The external surface, convex and smooth, articulates with the 

 trapezium. The internal surface is concave and smooth in front, for articulation 

 with the os magnum ; rough behind, for the attachment of an interosseous 

 ligament. 



Hold the bone with the larger, non-articular surface toward you, and the 

 smooth, quadrilateral articular surface upward. The convex, articular surface 

 will point to the side to which the bone belongs. 1 



Articulations. With four bones : the scaphoid above, second metacarpal bone 

 below, trapezium externally, os magnum internally. 



Os MAGNUM (Fig. 112). 



The Os Magnum is the largest bone of the carpus, and occupies the centre of 

 the wrist. It presents, above, a rounded portion or head, which is received into 



semilunar. For semi ] unar _ 



For scaphoid.^ ^sj 



For unciform.^ fff 



For 4th 

 metacarpal. - 



For 3d For 4th metacarpal. Ant. surface, 



metacarpal. 



FIG. 112. The left os magnum. 



the concavity formed by the scaphoid and semilunar bones ; a constricted portion 

 or neck ; and, below, the body. The superior surface is rounded, smooth, and 

 articulates with the semilunar. The inferior surface is divided by two ridges into 

 three facets, for articulation with the second, third, and fourth metacarpal bones, 

 that for the third (the middle facet) being the largest of the three. The posterior 

 or dorsal surface is broad and rough ; the anterior or palmar, narrow, rounded, 

 and also rough, for the attachment of ligaments and a part of the Adductor 

 obliquus pollicis. The external surface articulates with the trapezoid by a small 

 facet at its anterior inferior angle, behind which is a rough depression for the 

 attachment of an interosseous ligament. Above this is a deep and rough groove, 

 which forms part of the neck and serves for the attachment of ligaments, bounded 

 superiorly by a smooth, convex surface, for articulation with the scaphoid. The 

 internal surface articulates with the unciform by a smooth, concave, oblong facet 

 which occupies its posterior and superior parts, and is rough in front, for the 

 attachment of an interosseous ligament. 



Hold the bone with the broader, non-articular surface toward you, and the 

 head upward. The small, articular facet at the anterior inferior angle of the 

 external surface will point to the side to which the bone belongs. 



Articulations. With seven bones : the scaphoid and semilunar above ; the 

 second, third, and fourth metacarpal below ; the trapezoid on the radial side ; and 

 the unciform on the ulnar side. 



Attachment of Muscles. Part of the Adductor obliquus pollicis. 



Occasionally in a badly marked bone there is some difficulty in ascertaining to which side the 

 bone belongs ; the following method will sometimes be found useful : Hold the bone with its broader, 

 non-articular surface upward, so that its sloping border is directed toward you. The border will slope 

 to the side to which the bone belongs. 



