THE CARPUS 199 



by a prominent ridge, the oblique ridge of the trapezium (tubercidum ossis itnil- 

 tanguli major is). This surface gives origin to the Abductor pollicis, Opponens 

 pollicis, deep head of the Flexor brevis pollicis muscles, and the anterior annular 

 ligament. The posterior or dorsal surface is rough. The external surface is also 

 broad and rough, for the attachment of ligaments. The internal surface presents 

 two articular facets; the upper one, large and concave, articulates with the trape- 

 zoid; the lower one, small and oval, with the base of the second metacarpal bone. 



To ascertain to which hand this bone belongs, hold it with the grooved palmar surface upward, 

 and the external broad, nonarticular surface backward; the saddle-shaped surface will then be 

 directed to the side to which the bone belongs. 



Articulations. With four bones the scaphoid above, the trapezoid and second metacarpal 

 bones internally, the first metacarpal below. 



Attachment of Muscles. Abductor pollicis, Opponens pollicis, and deep head of the Flexor 

 brevis pollicis. 



The trapezoid (os multangulum minus) (Fig. 158) is the smallest bone in the 

 second row. It may be known by its wedge-shaped form, the broad end of the 

 wedge forming the dorsal, the narrow end the palmar, surface, and by its having 

 four articular surfaces touching each other and separated by sharp edges. 



Palmar For 

 For scaphoid surface trapezium 



For 2nd 

 metacarpal 



B 



FIG. 158. The left trapezoid. A, as seen from above, inner side and behind; B, from in front, 



below and outer side. 



Surfaces. The superior surface, quadrilateral in form, smooth, and slightly con- 

 cave, articulates with the scaphoid. The inferior surface articulates with the upper 

 end of the second metacarpal bone; it is convex from side to side, concave from 

 before backward, and subdivided by an elevated ridge into two unequal lateral 

 facets. The posterior or dorsal and anterior or palmar surfaces are rough, for the 

 attachment of ligaments, the former being the larger of the two. The anterior 

 surface gives origin to the deep head of the Flexor brevis pollicis. The external 

 surface, convex and smooth, articulates with the trapezium. The internal surface 

 is concave and smooth in front, for articulation with the os magnum; rough behind, 

 for the attachment of an interosseous ligament. 



To ascertain to which hand this bone belongs, let the broad dorsal surface be held upward, 

 and the inferior concavo-convex surface forward; the internal concave surface will then point 

 to the side to which the bone belongs. 



Articulations. With/owr bones the scaphoid above, second metacarpal bone below, trape- 

 zium externally, os magnum internally. 



Attachment of Muscles. To the deep head of the Flexor brevis pollicis. 



The os magnum (os capitatum) (Fig. 159) is the largest bone of the carpus, 

 and occupies the centre of the wrist. It presents, above, a rounded portion or 

 head, which is received into the concavity formed by the scaphoid and semilunar 

 bones; a constricted portion or neck; and, below, the body. 



Surfaces. The superior surface is rounded, smooth, and articulates with the semi- 

 lunar. The inferior surface is divided by two ridges into three facets for articu- 

 lation with the second, third, and fourth metacarpal bones, that for the third (the 



