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 pointed projection, the processus anconzus 
or “beak,’’ which overhangs the semilunar 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 are two convex facets which 
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 
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- 
Fig. 69.—Distat Enp or Lerr Rapius anp Utna oF Horse; Enp Fic. 70.—Saairrat Section oF UPPER 
View. Parr oF Rapius anpD ULNA OF HORSE. 
1, 2, 3, 4, Facets which articulate with radial, intermediate, ulnar, Cm, Medullary cavity of ulna. 
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. 
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 rows, proximal or antibrachial and distal or metacarpal. The 
1 Formerly called the sigmoid cavity. 
