628 MAMMALIAN ANATOMY 



THE THIRD METACARPAL BONE. 



General Description. The third metacarpal is about one-sixth 

 longer and somewhat stouter than the second. It is slightly longer 

 but not stouter than the fourth. 



The Base, unlike the base of the second, is directly continuous 

 with the shaft, and lies in the long axis of the entire bone. It may 

 be distinguished from the bases of all the others by the shape of its 

 proximal surface (Fig. 487), which is L-shaped and consists of two 

 parts : (1) a rectangular portion lying on the ulnar side and extending 

 from the dorsal to the palmar aspect, and (2) a sharp triangular part 

 directed transversely from the dorsal end of the first part. 



The first or ulnar part is rather more than twice as long in its 

 dorso-palmar diameter as it is wide from side to side. Its dorsal 

 border is rounded and its ulnar border is sinuate, arcuate dorsally 

 and then faintly ernarginate ; it terminates at its palmar end by being- 

 directed obliquely to the radial and palmar sides and meeting the 

 radial border in a point. The radial border begins behind at the 

 palmar angle and is directed for a distance obliquely dorsally and to 

 the radial side. It then bends at an angle and runs directly forward 

 until it reaches the base of the triangular second part, where it may be 

 said to be continued forward as a groove which separates the two parts 

 and finally ends in the rounded dorsal border. This first part of the 

 proximal surface is convex from before backward. In the transverse 

 direction it is convex at first on the ulnar side and then becomes 

 slightly concave where it approaches the triangular second part. It 

 does not face directly proximally, but is inclined to the radial side. 

 It is covered with cartilage, and articulates with a facet on the distal 

 surface of the magnum. Its radio-palmar portion is slightly over- 

 lapped by the distal surface of the hook-like palmar part of the base 

 of the second metacarpal. 



The second or radial part appears as a process of the first part, 

 and is separated from it by a notch in front and by a slight groove 

 above. The groove runs obliquely backward and to the ulnar side, 

 and is wider in front than behind, so that the process appears as a tri- 

 angle with its longer side on the palmar aspect and the other two 

 nearly equal sides directed forward to the dorsal and transversely to 

 the ulnar side respectively. Its surface is slightly convex in both 

 directions, and faces to the radial side as well as to the proximal side ; 



