Limbs : Upper Limb 



109 



of superficial fibres of the anterior annular ligament, and this overhangs a well-marked 

 groove containing tendon of F. carpi radialis and bounded below by a smaller ridge 

 for deeper fibres of the ligament : thus the tendon might be described as passing 

 through the outer attachment of the annular ligament (see Fig. 84). The surface 

 carrying the groove looks inwards and forwards towards the palm, and the groove 

 is directed towards the base of the index metacarpal. 



The postero-external surface is roughened by ligaments and supports the radial 

 artery as it runs to the first intermetacarpal space. 



Trapezoid. Difference in size between anterior and posterior surfaces is well 

 marked : increase in dorsal surface particularly in a downward and inward direction, 

 while the rough anterior surface is drawn into a lower and outer point, fitting as a 

 roughened angle between the facets for trapezium and index metacarpal (Fig. 88). 



This bone only supports one metacarpal, and its lower surface is moulded into a 

 concavo-convex form to fit the deeply-grooved base of this bone. Outer facet for 

 trapezium is convex, and upper one for scaphoid is slightly concave, and these two 

 are directly continuous, as is the upper one with that on the inner side for os magnum, 



SUfkoiel 



Txift>$oid 

 '"tetuifil 



AfmtZuiur 



Unei/orm 



Dorsal 

 Surface 



l^neTiar/ai 



FIG. 89. Left os magnum. From the outer and inner sides. To tell left 

 from right, hold the round head proximally, the larger non-articular 

 surface dorsally, and the abruptly flattened side inwards. 



and this again with the lower facet. In fact all the facets form a continuous articular 

 surface round the bone, only partly interrupted in front and externally by the pointed 

 prolongation of the anterior surface. 



Os Magnum. A large dorsal surface coming obliquely to a point directed down- 

 wards and inwards and narrowing proximally to give the appearance of a neck sup- 

 porting the upper articular head : front surface smaller, irregular, roughened for liga- 

 ments. Outer surface with two facets, partly continuous, for scaphoid and trapezoid, 

 but with a hollow area between them on the palmar side for an interosseous ligament. 

 Inner surface rather flattened, with facet for unciform extending along its posterior 

 part, but roughened in front of this for interosseous ligament : this is the strongest 

 of these ligaments in the hand, and has the largest surface of attachment. 



The lower surface has a main facet for the middle metacarpal, and on each side 

 of this a smaller one for the neighbouring metacarpals : that for the fourth metacarpal 

 is small and confined to the back part of the surface, for the front portion is taken up 

 by the large interosseous band, which not only ties the os magnum to the unciform, 

 but also to the fourth metacarpal. 



The obliquity of the distal border of the dorsal surface is a consequence~oi the 

 presence of the styloid process on the middle metacarpal, and the dorsal surface may 

 present a bevelled-off portion to receive this. 



