THOKACIC LIMBS 631 



The Head differs from the head of the second metacarpal in being 

 transversely symmetrical. 



Determination. The third metacarpal can be easily recognized 

 by its L-shaped proximal surface. When the bone is held with the 

 dorsal surface upward and the proximal surface toward the student, 

 the prominent hook-like projection on the ulnar side of the base points 

 to the side to which the bone belongs. 



Articulation. The third metacarpal articulates with the magnum 

 above, with the unciform and fourth metacarpal on the ulnar side, with 

 the second metacarpal on the radial side, and with its own proximal 

 phalanx below. 



Muscular Attachments. On the dorsal surface of the base is 

 attached the extensor carpi radialis brevior ; to the palmar border, 

 the second palmar interosseous ; and to the radial and ulnar sides 

 of the shaft, the second and the third dorsal interosseous respectively. 



Ossification. The third metacarpal is developed from two 

 centres of ossification, one for the shaft and one for the head. 



THE FOURTH METACARPAL BONE. 



General Description. The fourth metacarpal resembles the 

 third in many particulars ; it is very little shorter, and possibly some- 

 what stouter. 



Its Base presents the distinguishing characteristics. As in the 

 third metacarpal, the proximal surface consists of two parts, a rec- 

 tangular ulnar part and a triangular radial part. 



The ulnar part is three times as long as it is wide, and convex 

 from before backward ; it slopes slightly to the radial side and articu- 

 lates with the distal surface of the unciform and at the radio-palmar 

 angle slightly with the magnum and the third metacarpal. The radial 

 part is joined to the dorsal half of the radial side of the ulnar part, 

 and is directed obliquely to the dorsal and radial sides. It is convex 

 in both directions, and separated from the first part by a groove which 

 is wider in front than behind ; it articulates with the ulnar surface of 

 the base of the third metacarpal. 



The proximal surface as a whole differs from that of the third 

 metacarpal in possessing a truncate palmar border and a straight ulnar 

 border, and in sloping without any distinct dorsal border into the 

 dorsal surface of the shaft. While the highest part of the proximal 



