THOEACIC LIMBS 



611 



FIG. 474. 



the dorsal surface toward the student, the straight end of the ridge on 

 the proximal surface points to the side to which the bone belongs. 



Articulation. The trapezoid articulates with the scapho-lunar 

 above, with the magnum on the inside, with the second metacarpal 

 below, and with the trapezium on the outside. Its palmar angle does 

 not show on the palmar surface of the hand ; the trapezium and 

 magnum come together and exclude it. 



Ossification. The trapezoid develops from one centre of ossifi- 

 cation. 



Muscular Attachment. The palmar surface of the trapezoid 

 affords attachment to ligaments to which is attached the origin of the 



flexor brevis pollicis. 



THE MAGNUM. 



General Description. The Magnum, or os magnum, is the 

 central bone of the distal row of the carpus, and lies between the 

 trapezoid and the unciform, above the third and 

 fourth metacarpals, and below the scapho-lunar 

 (Fig. 474). In shape it is very irregular, but for 

 purposes of description may be divided into two 

 parts, a thin horizontal plate, or body, and a ver- 

 tical plate, or head. The horizontal plate, or body, 

 is rectangular, twice as long in the dorso-palmar 

 direction as it is wide in the radio-ulnar diameter. 



The vertical plate, or head, is placed at right 

 angles to the body, but attached obliquely across its 

 proximal surface, so that its dorsal end is at the 

 ulnar side and its palmar end at the radial side, 

 thus exposing two triangles of the proximal surface 

 of the body, one at the radio-dorsal angle and 

 another at the ulno-palmar angle. The proximal 

 border of the head is strongly arched from before 

 backward. 



The magnum as a whole presents on examina- 

 tion six surfaces, proximal, dorsal, radial, ulnar, palmar, and distal. 



The proximal surface (Fig. 475) of the bone consists of three dis- 

 tinct parts : (1) the radio-dorsal triangle of the body mentioned above, 

 on the outer side of which is a small square facet for articulation with 

 the dorsal end of the oblique ridge on the distal surface of the scapho- 



POSITION OF MAGNUM. 



