THE BONES OF THE UPPER LIMB 



205 



carpal bone. Both of these surfaces are rough for ligaments. 

 The superior surface presents a quadrilateral, concave facet, elon- 

 gated from before backwards, for the scaphoid. The inferior 

 surface is characterized by a large saddle-shaped facet, convex 

 from side to side and concave from before backwards, for the base 

 of the second metacarpal bone. The external surface has a convex 

 facet for the trapezium, below which there is a rough triangular 

 surface with the base directed anteriorly. The internal surface is 

 concave from before backwards, and its anterior portion presents 

 a facet for the os magnum. 



For Trapcziain 

 (on External Surface) 



/ Superior Surface 



for Scaphoid 



^"^k External Sor&ce 



~~^ Part of Inferior Surface 

 for snd Metacarpal 



Dorsal Snrface 



Part of Inferior Surface 

 for 2nd Metacarpal 



-Internal Surface 

 for Os Magnum 



L.' Dorsal Surface 



Fio. 118. — The Right Trapezoid Bone. 

 A, External View ; B, Posterior View. 



Articulations. — Superiorly with the scaphoid, inferiorly with the 

 second metacarpal, externally with the trapezium, and internally 

 with the OS magnum. 



The Os Magnum. 



The OS magnum is the largest bone of the carpus, its distinctive 

 characters being that it is composed of a head, neck, and bod}'. The 

 superior and external aspects of the head are convex, and merge 

 gradually into each other. The cartilage of the superior aspect is 

 prolonged more behind than in front, and articulates with the semi- 

 lunar. The external aspect of the head articulates with the scaphoid. 

 The internal aspect of the head is flat, and presents the commence- 

 ment of the facet for the unciform bone. The neck is mainly present 

 in front and behind. 



The body is quadrilateral, and narrower in front than behind. 

 The palmar and dorsal surfaces are rough, the former giving origin 

 to fibres of the adductor obliquus poUicis. The external surface, 

 which is con inuous with the outer convex aspect of the head, 

 presents anteriorly a facet for the trapezoid. The internal surface 

 presents at its back part the lower portion of the facet for the 

 unciform, which is here narrow. The inferior surface is narrow in 

 front, but broad behind, and the internal of the two posterior 

 angles is elongated downwards and inwards. This surface presents 



