THE CARPUS 



93 



continued by a fibrous cord to the distal tuberosity of the radius, but this band may 

 be replaced in part or entirely by bone. 



The proximal extremity (Extremitas proximalis) is the major part of the bone. 

 It projects upward and somewhat backward behind the distal end of the humerus, 

 and forms a lever arm for the extensor muscles of the elbow. The medial surface 

 is concave and smooth. The lateral surface is convex and is roughened above. 

 The dorsal border bears on its middle a pointefl projection, the processus anconeus 

 or "beak, " which overhangs the semilimar notch 

 (Incisura semilunaris). ^ The latter is triangular 

 in outline, concave from above downward, convex 



transversely, and articulates with the humerus; in ' \ 



its lower part there is an extensive synovial fossa. '' 



Just below the notch PTe two convex facets which ,J} 



articulate with those on the volar aspect of the 

 proximal end of the radius. The volar border is 

 nearly straight, and is thick and rounded. The 

 free end or summit is a rough tuberosity, the ole- 

 cranon, which gives attachment to the triceps 

 brachii and other muscles. 



The primitive distal extremity has, as pre- 

 viously stated, fused with the radius. 



Development. — The ulna ossifies from three 

 centers, of which one is for the main part of the ^ -.-^ 

 bone, one for the olecranon, and one for the distal ;, ' 



end. The cartilaginous embryonic ulna extends '^i, 



the entire length of the forearm. The distal part ' ■[ 



of the shaft is usually reduced to a small fibrous 

 band or may disappear entirely; in some cases a 

 variable remnant of it ossifies. The distal ex- Vlf 



'■>M 



? 



\ 



^'g'f^f li 



!^.i 



Fig. 69. — Distal End of Left Radius and Ulna of Horse; End 

 View. 

 1, 2, 3, 4, Facets which articulate with radial, intermediate, ulnar, 

 and accessory carpal bones respectively; 5, groove for tendon of exten- 

 sor carpi radialis; 6, groove for tendon of common digital extensor; 7, 

 tuberosity for attachment of medial ligament of carpal joint. 



Fig. 70. — Sagittal Section of Uppeb 



Part of Radius and Ulna of Horse. 



Cm, ^Medullary cavity of ulna. 



tremity fuses before birth with the radius. The olecranon unites with the rest 

 of the bone at about three and a half years. A medullary cavity appears to occur 

 constantly in the adult — contrary to the statements of some authors. 



THE CARPUS 



The carpus of the horse consists of seven or eight carpal bones (Ossa carpi) 

 arranged in two rowSj proximal or antibrachial and distal or metacarpal. The 



^ Formerly called the sigmoid cavity. 



