THE CARPUS — THE INTERMEDIATE CARPAL BONE 



83 



articulates with the humerus; in the middle of its lower part is an extensive 

 synovial fossa. Just below the notch are two convex facets which articulate with 

 those on the posterior aspect of the proximal end of the radius. The posterior 

 border is nearly straight, and is thick and rounded. The free end or sunnnit is 

 a rough tuberosity, the olecranon, which gives attachment to the triceps brachii 

 and other muscles. 



The primitive distal extremity has, as previously 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 lower part of the -j-'^^;.-'- 



shaft is usually reduced to a small fibrous band - " >' 



or may disappear entirely ; in some cases a vari- 

 able remnant of it ossifies. The distal extremity 



fuses early with the radius. The olecranon unites • * 



with the rest of the bone at three to three and a -.'/.,',' 



half years. A medullary canal appears to occur ' .',' '// 



constantly in the adult — contrary to the state- ,;- ' ' 



ments of some authors. 







THE CARPUS 



The carpus of the horse consists of seven or 

 eight bones (Ossa carpi) arranged in two rows, 

 proximal or antibrachial, and distal or metacar- 

 pal. The (abbreviated) names and relative posi- 

 tions of the bones of the left carpus as seen from 

 in front are indicated below. 



Proximal Row: 

 Radial Intermediate Ulnar Accessory 



Distal Rou\- 

 First Second Third Fourth 



The Radial Carpal Bone 



The radial carpal bone (Os carpi radiale, 

 scaphoid) is the largest bone of the upper row; it 

 is somewhat compressed laterally, and is clearly 

 six-sided. The superior or proximal surface is 

 convex in front, concave behind, and articulates 

 with the inner facet on the distal end of the radius. 

 The inferior or distal surface is also convex in front 

 and concave behind; it articulates with the second 



and third carpal bones. The external surface bears upper and lower facets on its 

 anterior part for articulation with the intermediate ; between and behind these it is 

 excavated and rough. The anterior or dorsal surface is rough and slightly convex. 

 The internal surface and the posterior or volar surface are rough and tuberculate. 



'V^ 



Fig. .51. — Sagittal Section of Upper 



Part of Radius and Ulna of Horse. 



Cm, Medullary cavity of ulna. 



The Intermediate Carpal Bone 

 The intermediate carpal bone (Os carpi intermedium, semilunar, lunar) is 

 somewhat wedge-shaped, wider in front than behind. The superior or proximal 

 surface is saddle-shaped, and articulates with the middle facet on the distal end of 



