608 



MAMMALIAN ANATOMY 



FIG. 471. 



above, the trapezoid and second metacarpal in front and within, and 

 the first metacarpal without and behind. It is attached by its palmar 

 surface to the palmar end of the vertical plate of the magnum. 



Ossification. The trapezium is developed from one centre of 



ossification. 



THE TRAPEZOID. 



General Description. The trapezoid is the second bone on the 

 radial side of the distal row (Fig. 471). Next to the trapezium it is 



the smallest bone of the carpus. It may be easily 

 distinguished by its small size and compressed, 

 plate-like form. It lies distal to the scapho-lunar 

 and on the ulnar side of the trapezium. When 

 seen from above or below, its outline is that of a 

 square with one angle directed to the palmar side, 

 with the opposite dorsal angle rounded and the 

 ulnar angle truncated. 



It exhibits nearly flat and parallel proximal 

 and distal surfaces, a straight radio-palmar sur- 

 face, which forms right angles with the straight 

 and parallel radio-dorsal and palmo-ulnar sur- 

 faces, a convex dorsal surface, which meets the 

 radio-dorsal surface and the sharp dorso-ulnar 

 border at angles of forty-five degrees, and, 

 finally, a dorso-ulnar surface, formed by the 

 cutting off of the ulnar angle of the square and 



POSITION OF TRAPEZOID. 



converted into a sharp border by the sloping 



downward of the proximal surface and the turning 'up of the distal 

 surface. This border forms angles of forty-five degrees with the 

 dorsal and palmo-ulnar surfaces. 



The proximal surface, which exhibits the general outline of the 

 bone, is divided into two parts by a ridge running parallel to the 

 radio-dorsal and palmo-ulnar borders until it reaches the dorsal third 

 of the surface, where it curves to the ulnar side in a distal and palmar 

 direction to meet the angle formed by the meeting of the dorsal surface 

 and dorso-ulnar border. 



The outer of these parts is smaller, narrower, and more elevated 

 than the inner. It is principally occupied by an elongated, crescentic, 

 articular facet, which is gently concave from side to side and joins a 



