BONES OF THE THORACIC LIMB 



201 



to sWe on.v; T "IT 'k*'?'"' T^^'' ^°™^^1 by them is concave from side 



sur act; ofX. r ^TX^'fTl'^i ^^^ ^'«*^1 ^"^s (Capitula) have articular 



cent ?hp 11 wT ^ ^'^^' ^;^* ^'^' ^ '""^^^^^ "dg*^ °n the volar aspect, c-x- 

 cept the first, which is grooved. Ossification is complete at five or six months of 



TheThw^nitL'hito'r^f Sr" """■ ■"'"'" ""= "-'■ •"»■> '- '-• 



longest; the first is very short and does 

 not come in contact with the ground 

 in walking. The first phalanges of the 

 chief digits have four-sided shafts, 

 which are slightly curved dorsally. 

 The proximal end of each has a con- 

 cave surface for articulation with the 

 metacarpal bone and is deeply notched 

 behind. The distal end has a trochlea 

 for articulation with the second pha- 

 lanx, and depressions on each side for 

 ligamentous attachment. The second 

 phalanges are about two-thirds of the 

 length of the first phalanges. The 

 proximal articular surface consists of 

 two cavities separated by a sagittal 

 ridge. The distal extremity is wider 

 and flatter than that of the first. The 

 third phalanges correspond in general 

 to the form of the claws. The base has 

 an [articular surface adapted to [ the 

 second phalanx and is encircled by a 

 collar of bone (Crista unguicularis). 

 The volar surface bears a wing or tuber- 

 osity, and on each side of this is a fora- 

 men. The ungual part is a curved rod 

 with a blunt-pointed free end. It-is 

 rough and porous. Its base forms with 

 the collar previously inentioned a deep 

 groove, into which the proximal border 

 of the claw is received. The two pha- 

 langes of the first digit resemble in ar- 

 rangement the first and third phalanges 

 of the other digits. Ossification is 

 complete at five or six months. 



Nine volar sesamoids are usually 

 present. Two are found at each meta- 

 carpo-phalangeal joint of the chief 

 digits. They are high and narrow, 

 articulate Avith the distal end of the 



metacarpal bone in front, and have a small facet on the base for the first phalanx. 

 On this joint of the first digit there is usually a single flattened sesamoid, but ex- 

 ceptionally two are present. The distal volar sesamoids remain cartilaginous. A 

 nodular dorsal sesamoid occurs in the capsule of the metacarpo-phalangeal joints, 

 and cartilaginous nodules are found in a similar position in connection with the 

 joints between the first and second phalanges. 



Fig. 221. — Skeleton of Distal Paet of Right Thoracic 

 Limb of Dog; Dorsal View. 

 The digits are spread. /, Distal end of interosseous 

 space: R, distal end of radius; C. r.+ i., radio-intermediate 

 carpal; C. »., ulnar carpal; C. a., accessory carpal (very 

 small part visible) ; C. 1, C. 2, C. 3, C. 4-, first to fourth car- 

 pal bones; Mc. I, metacarpal bone of first digit; P. 1+2, 

 fused first and second phalanges of same ; P. S, third phalanx 

 of same; Mr. V, fifth metacarpal bone; P. /, P. 2, P. 3, 

 phalanges of fifth digit ;~5, dorsal sesamoid; C, C, volar 

 sesamoids. 



