202 



SPECIAL ANATOMY OF THE SKELETON 



convex from before backward, less so from side to side, and flattened laterally; 

 it articulates with the proximal phalanx; it is broader and extends farther forward 

 on the palmar than on the dorsal aspect. It is longer in the antero-posterior than 

 in the transverse diameter. On either side of the head is a tubercle for the attach- 

 ment of the lateral ligament of the metacarpophalangeal joint. The posterior 

 surface, broad and flat, supports the Extensor tendons; the anterior surface is 

 grooved in the middle line for the passage of the Flexor tendons, and marked on 

 each side by an articular eminence continuous with the terminal articular surface. 



The metacarpal spaces (spatia interossea metacarpi) 

 are the intervals between the metacarpal bones. 

 They are occupied by the Interossei muscles. The 

 broadest space is between the metacarpal bones of 

 the thumb and index finger. 



Peculiar Characters of the Metacarpal Bones. 

 -The metacarpal bone of the thumb (os metacar- 

 pale 7) (Fig. 161) is shorter and wider than the 

 rest, diverges to a greater degree from the carpus, 

 and its palmar surface is directed inward toward 

 the palm. The shaft is flattened and broad on 

 its dorsal aspect, and does not present the ridge 

 which is found on the other metacarpal bones; it 

 is concave from above downward, on its palmar 

 surface. The carpal extremity, or base, presents 

 a concavo-convex surface, for articulation with the 

 trapezium; it has no lateral facets, but presents 

 externally a tubercle for the insertion of the Extensor ossis metacarpi pollicis. 

 The distal extremity is less convex than that of the other metacarpal bones, broader 

 from side to side than from before backward. It presents on its palmar aspect 

 two distinct articular eminences for the two sesamoid bones in the tendons of 

 the Flexor brevis pollicis, the outer one being the larger of the two. 



Tubercle. 

 For trapezium. For trapezium. 



FIG. 161. The first metacarpal. (Left.) 



The side to which this bone belongs may be known by holding it in the position it occupies 

 in the hand, with the carpal extremity upward and the dorsal surface backward; the tubercle for 

 the Extensor ossis metacarpi pollicis will point to the side to which it belongs. 



Attachment of Muscles. To four the Opponens pollicis, the Extensor ossis metacarpi 

 pollicis, the Flexor brevis pollicis, and the First dorsal interosseous. 



The metacarpal bone of the index finger (os metacarpale 77) (Fig. 162) is the 

 longest and its base the largest of the other four. Its carpal extremity is prolonged 

 upward and inward, forming a prominent ridge. The dorsal and palmar surfaces 

 of this extremity are rough, for the attachment of tendons and ligaments. It pre- 

 sents four articular facets three on the upper aspect of the base; the middle 

 one of the three is the largest, concave from side to side, convex from before back- 

 ward, for articulation with the trapezoid; the external one is a small, flat, oval 

 facet, for articulation with the trapezium; the internal one on the summit of the 

 ridge is long and narrow, for articulation with the os magnum. The fourth facet 

 is on the inner or the ulnar side of the extremity of the bone, and is for articulation 

 with the third metacarpal bone. 



The side to which this bone belongs is indicated by the absence of the lateral facet on the outer 

 (radial) side of its base, so that if the bone is placed with the base toward the student and the 

 palmar surface upward, the side on which there is no lateral facet will be that to which it belongs. 



Attachment of Muscles. To six Flexor carpi radialis, Extensor carpi radialis longior, 

 Abductor obliquus pollicis, First and Second dorsal interosseous, and First palmar interosseous. 



