146 OSTEOLOGY. 



THE METACAKPUS. 



The Metacarpal bones are five in number : they are long cylindrical bones, 

 presenting for examination a shaft, and two extremities. 



COMMON CHAEACTEES OF THE METACAEPAL BONES. 



The shaft is prismoid in form, and curved longitudinally, so as to be convex in 

 the longitudinal direction behind, concave in front. It presents three surfaces : 

 two lateral, and one posterior. The lateral surfaces are concave, for the attach- 

 ment of the Interossei muscles, and separated from one another by a prominent 

 line. The posterior or dorsal surface is triangular, smooth, and flattened below, 

 and covered, in the recent state, by the tendons of the extensor muscles. In its 

 upper half, it is divided by a ridge into two narrow lateral depressions, for the 

 attachment of the Dorsal interossei muscles. This ridge bifurcates a little above 

 the centre of the bone, and its branches run to the small tubercle on each side of 

 the digital extremity. 



The carpal extremity or base is of a cuboidal form, and broader behind thnn in 

 front: it articulates, above, with the carpus; and,' on each side, with the adjoining 

 metacarpal bones ; its dorsal and palmar surfaces being rough, for the attachment 

 of tendons and ligaments. 



The digital extremity or head presents an oblong surface, flattened at each 

 side, for articulation with the first phalanx; it is broader and extends farther 

 forwards in front than behind ; and is longer in the antero-posterior than in the 

 transverse diameter. On either side of the head is a deep depression, surmounted 

 by a tubercle, for the attachment of the lateral ligament of the metacarpo-phalan- 

 geal joint. The posterior surface, broad and flat, supports the Extensor tendons ; 

 and the anterior surface presents a median groove, bounded on each side by a 

 tubercle, for the passage of the Flexor tendons. 



PECULIAB METACAEPAL BONES. 



The metacarpal bone of the thumb is shorter and wider than the rest, diverges 

 to a greater degree from the carpus, and its palmar surface is directed inwards 

 towards the palm. The shaft is flattened and broad on its dorsal aspect, and does 

 not present the bifurcated ridge peculiar to the other metacarpal bones ; concave 

 from before backwards on its palmar surface. The carpal extremity or base 

 presents a concavo-convex surface, for articulation with the trapezium, and has no 

 lateral facets. The digital extremity is less convex than that of the other metacarpal 

 bones, broader from side to side than from before backwards, and terminates anteri- 

 orly in a small articular eminence on each side, over which play two sesamoid bones. 



The metacarpal bone of the index finger is the longest, and its base the largest of 

 the other four. Its carpal extremity is prolonged upwards and inwards ; and its 

 dorsal and palmar surfaces are rough, for the attachment of tendons and ligaments. 

 It presents four articular facets: one at the end of the bone, which has an angular 

 depression, for articulation with the trapezoid ; on the radial side, a flat quadri- 

 lateral facet, for articulation with the trapezium ; its ulnar side being prolonged 

 upwards and inwards, to articulate, above, with the os magnum ; internally, with 

 the third metacarpal bone. 



The metacarpal bone of the middle finger is a little smaller than the preceding ; 

 it presents a pyramidal eminence on the radial side of its base (dorsal aspect), which 

 extends upwards behind the os magnum. The carpal articular facet is concave be- 

 hind, flat and horizontal in front, and corresponds to the os magnum. On the radial 

 side is a smooth concave facet, for articulation with the second metacarpal bone ; and 

 on the ulnar side two small oval facets, for articulation with the fourth metacarpal. 



The metacarpal bone of the ring finger is shorter and smaller than the pre- 

 ceding, and its base small and quadrilateral, its carpal surface presenting two 

 facets, for articulation with the unciform and os magnum. On the radial side 

 are two oval facets, for articulation with the third metacarpal bone ; and on the 

 ulnar side a single concave facet, for the fifth metacarpal. 



