610 



MAMMALIAN ANATOMY 



slightly to the radial side as well as upward. The proximal border 

 has a concavity on its ulnar half, the end of the concave articular 

 facet on the proximal surface, and slopes downward and outward to 

 the radial angle. The distal border is straight and parallel with the 

 corresponding part of the proximal border for a short distance from 

 the ulnar side ; it then curves upward and is lost as this surface passes 

 into the distal surface. The entire radio-palmar surface is smooth, 

 and covered with cartilage for articulation with a facet on the inner 

 surface of the trapezium. 



The distal surface (Fig. 473) has the same general outline as the 

 proximal surface. It exhibits a central oval surface, which is gently 



FIG. 473. 

 Dorsal Side. 



Radial Side. 



With Second 

 Metacarpai. ~ 



With Magnum. 



Ulnar Side. 



With Trapezium. 



Palmar Side. 



LEFT TRAPEZOID, DISTAL SURFACE. 



concave for articulation with the base of the second metacarpal. This 

 surface is bounded in front and on the radial side by the curved dorsal 

 and radio-dorsal borders. A straight, low ridge cuts off along the 



* 



dorso-ulnar border a small articular facet which slopes upward and 

 inward ; it articulates with a facet on the dorsal end of the radial 

 side of the proximal surface of the process on the horizontal plate 

 of the magnum. The palmo-ulnar border of the larger facet runs 

 to the radial and palmar sides and meets the radio-palmar border 

 at a right angle. The latter border is straight for about half its 

 length, and then ends at a pit for the insertion of a ligament from 

 the trapezium. At this point the distal surface curves abruptly 

 upward into the radio-palmar surface and forms part of the articular 

 surface for the trapezium. 



Nomenclature. Trapezoid is formed by the union of trapezium, 

 and eides, like. Its common synonyms are Os muUangulum minus, 

 carpale II., das kleine vieleckige Bein, das Trapezoidbein, le trapezo'ide. 



Determination. When the proximal surface is held upward and 



