THOR.ACIC LIMBS. 71 



oblong plate bearing on its proximal surface a semicircular 

 ridge which crosses it diagonally. The proximal end of the 

 bone articulates with the scapholunar. Its distal end articulates 

 with the third metacarpal except near its ventroulnar angle, 

 where it articulates with the fourth metacarpal. Its ulnar sur- 

 face articulates with the unciform. Its radial border articulates 

 with the trapezoid, the third metacarpal, and, by two facets, 

 with the second metacarpal. 



Trapezoid. {Os nniltangulum viinus BNA) (Fig. 51, e). — 

 The trapezoid is somewhat wedge-shaped, with the apex of the 

 wedge pointing ventrad. Its proximal side articulates with the 

 scapholunar, its distal side with the second metacarpal, its ulnar 

 side w^ith the os magnum, and its radial side with the trapezium. 



Trapezium. {Os inultaiiguluni viajiis BNA.) (Fig. 51, d). 

 — The trapezium has the form of a triangular prism curved into 

 a semicircle. The convex face looks proximad and articulates 

 by its ventral half with the scapholunar. The ulnar surface 

 articulates with the second metacarpal dorsally, and ventrally 

 with the trapezoid. Its radial surface articulates with the first 

 metacarpal. 



Bones of the Hand or Manus (Fig. 51, i-r.j. — The Meta- 

 carpals. Metacarpus. — The metacarpals are the five bones of 

 the palm of the hand ; they are numbered from one to five, 

 beginning with the thumb. They are cylindical elongated bones 

 with enlarged ends. The distal end is called the head, and the 

 proximal end the base. Each head bears a hemispherical 

 articular facet which is marked over its ventral half by a 

 prominent smooth ridge. The surface dorsad of the ridge 

 articulates with the proximal head of a phalanx. The ridge 

 and the surface at its sides are for a pair of sesamoid bones. 



The first metacarpal ( 1 ) is the shortest. Its head is oblique, 

 and it articulates by the ulnar half of its proximal surface with 

 the trapezium {li) ; by the radial half with the radial sesa- 

 moid (Ji). 



The second metacarpal (2) is marked on the proximal part 

 of its dorsal surface by an oblique groove passing from the 

 radial side distad to the ulnar side. The base articulates with 

 the trapezoid {e). The ulnar surface of the proximal end 



