The =proximal extremity= is the major part of the bone. It projects upward and somewhat backward behind the lower end of the humerus, and forms a lever arm for the extensor muscles of the elbow. The =internal surface= is concave and smooth. The =external surface= is convex and is roughened above. The =anterior border= bears on its middle a pointed projection, the =processus anconæus= or “beak,” which overhangs the =semilunar notch= or =sigmoid cavity= (Incisura semilunaris). The latter is triangular in outline, concave from above downward, and 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 summit 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 shaft is usually reduced to a small fibrous band or may disappear entirely; in some cases a variable 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 statements 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 metacarpal. The (abbreviated) names and relative positions of the bones of the left carpus as seen from in front are indicated below. _Proximal Row:_ Radial Intermediate Ulnar Accessory _Distal Row:_ 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. [Illustration: