BONES OF THE THORACIC LIMB 



articulates with the ulnar carpal only. The first carpal is absent. The 



149 



second and 



:T.irwr,t 11^°"" ' ^^^^ quadrilateral bone. The fourth carpal 



is a 



third 



smaller quadrilateral bone. 



The metacarpus consists of a large metacarpal and a lateral small metacaroal 



and Wh^o^S TZl'f '°"!.^^^- ' +/^ ^'^^^^^^ ^-- *h^ fusion onheZd 

 ^ H + 7 ^u . . ^^'^^' """"^ ^^^^^ evidences of its double origin even in the 

 adult state The shaft is shorter than in the horse, and is reirtivdy^er and 

 flatter. The dorsa surface is rounded, and is marked by a vertiLrvascukr 

 groove c^miecting two canals which traverse the ends of the shaf f Inlb fore 

 backward. The volar surface is flat and presents a similar but much fainter groove 

 The borders are rough m the proximal third. The proximal end bears two si ghllv 

 concave facets for articulation with the bones of the lower row of the carpus the 

 medial area is the larger, and they are separated by a 

 ridge m front and a notch behind. The lateral angle 

 has a facet behind for the small metacarpal bone. The 

 medial part of the extremity has anterior and posterior 

 tuberosities. The distal end is divided into two parts 

 by a sagittal notch. Each division bears an articular 

 surface similar to that in the horse, but much smaller. 

 The medullary cavit\' is divided into two parts Ijy a ver- 

 tical septum which is usually incomplete in the adult. 

 The small metacarpal bone (Mc. 5) is a rounded rod 

 about an inch and a half (ca. 3.5 to 4 em.) in length, 

 which lies against the proximal part of the lateral bor- 

 der of the large bone. Its proximal end articulates with 

 the latter, but not with the carpus. The distal end is 

 pointed. 



Four caxtilaginous metacarpals are present in the early f cetal 

 state, VIZ., the second, third, fourth, and fifth. The second com- 

 monly either disappears or unites with the third; sometimes it 

 develops as a small rod of bone. The third and fourth gradually 

 unite, but can be cut apart at birth. Each has three centers of 

 ossification; the proximal epiphysis fuses with the shaft before 

 birth, the distal at two to two and one-half years. 



Four digits are present in the ox. Of these, two — 

 the third and fourth — are fully developed and have 

 three phalanges and three sesamoids each. The second 

 and fifth are vestiges and are placed behind the fetlock; 

 each contains one or two small bones which do not ar- 

 ticulate with the rest of the skeleton. 



The first phalanx is shorter and narrower than in 

 the horse and is three-sided. The interdigital surface is 

 flattened and its volar part bears a prominence for the 



attachment of the interdigital ligaments. The proximal extremity is relatively 

 large, and is somewhat compressed from side to side. The articular surface is con- 

 cave from before backward and is divided by a sagittal groove into two areas, of 

 which the abaxial one is the larger and higher. Behind these are two facets for 

 articulation with the sesamoid bones. The volar surface bears two tuberosities 

 separated by a deep depression. The distal extremity is smaller than the proximal, 

 especially in the dorso-volar direction. Its articular surface is divided by a sagittal 

 groove into two convex facets, of which the abaxial one is decidedly the larger. 

 There are depressions on either side for ligamentous attachment. The bone con- 

 sists at birth of two pieces — the distal end and the fused shaft and proximal ex- 

 tremity. Union occurs at one and one-half to two years. 



The second phalanx is about two-thirds of the length of the first and is dis- 



FiG. ISO. — Left Metacaepal 

 Bones of C)x; Front \iew. 

 The Small Bone has been 

 Moved Laterally. 



1, Metacarpal tuberosity: -- 

 vascular groove; 3, 3', foramina; 

 4, 4', condyles; 5, articular facet 

 of fifth (small) metacarpal bone. 



