THOKACIC LIMBS <i:25 



articular surface passes into the palmar surface of the shaft, in the 

 middle line, is a little sharp tubercle, and on either side a faint longi- 

 tudinal concavity for the base of the sesamoid bone. 



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Determination. When the dorsal side is held upward and the 

 proximal side toward the student, the long side of the bone is on the 

 side to which the bone belongs. 



Articulation. The metacarpal of the thumb articulates with the 

 trapezium above and its own proximal phalanx below, and is fastened 

 to the side of the second metacarpal by ligament. 



Muscular Attachments. The metacarpal of the thumb offers 

 attachment by the radial side of the base to the tendon of the extensor 

 ossis metacarpi pollicis, and by the palmo-ulnar surface to the first 

 dorsal interosseous muscle. 



Ossification. It is developed by two centres, one for the base 

 and one for the shaft. 



THE SECOND METACARPAL BONE. 



General Description. The second metacarpal is about three 

 times as long as the first, but very little stouter. 



The Base is prismatic in shape, broad in front and very narrow 

 behind. Its long axis is not in line with the long axis of the shaft, 

 but directed toward the ulnar side (Fig. 486). 



The inclination is made more apparent by the groove for the radial 

 artery, which is situated on the dorsal surface of the shaft ; it passes 

 obliquely downward from the radial to the ulnar side, and is continued 

 on the ulnar surface downward to the palmar side. The long dorso- 

 palmar diameter of the base is directed obliquely to the ulnar and 

 palmar sides. 



Its proximal surface (Fig. 487) is triangular, and convex from 

 before backward. From side to side it is at first slightly convex on 

 the ulnar side and then strongly concave from the middle to the radial 

 margin, the dorsal part of which rises to a sharp crest. This surface 

 articulates with the distal surface of the trapezoid. 



On the radial surface of the base (Fig. 492) near the external 

 border is an elongated oval facet for articulation with the facet on 

 the distal part of the ulnar surface of the trapezium. Behind this 

 facet the bone is rough for the attachment of ligaments and inter- 

 osseous muscles. 



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