The two bones of the forearm are relatively long and articulate with each other at either end in such a manner as to allow of slight movement. A narrow interosseous space separates their shafts. The =radius= is flattened from before backward and increases in size from above downward. The shaft forms two curves; one of these, an anterior convexity, involves the whole shaft; the other, an inner convexity, affects the upper part. The anterior surface is convex in both directions and is marked by a groove for the oblique extensor of the carpus. The posterior surface presents the nutrient foramen in its upper third, and bears a rough line (Crista interossea) externally for the attachment of the interosseous ligament. The proximal end (Capitulum radii) is relatively small and is supported by a distinct neck (Collum radii). It bears a concave surface (Fovea capituli) above for articulation with the humerus, and a convex marginal area (Circumferentia articularis) behind for the ulna. The bicipital tuberosity is small. There is a large external tuberosity and below this a rough eminence. The distal extremity is much wider. It has an extensive concave carpal articular surface. Its inner border projects downward, forming the =styloid process= of the radius. Externally there is a concave facet (Incisura ulnaris radii) for the ulna. In front are three distinct grooves for the extensor tendons. The =ulna= is well developed, but diminishes in size from above downward. It crosses the posterior surface of the radius from within outward. The shaft is large and three-sided in its upper two-thirds, smaller and more rounded below. Its anterior surface is in general rough. The nutrient foramen is near the proximal end. A vascular groove descends from it and indicates the course of the interosseous artery. The proximal end is relatively short. It is concave and smooth internally, convex and rough externally. The olecranon is grooved and bears three prominences, of which the posterior one is large and rounded. The semilunar notch is wide below and completes the surface for articulation with the trochlea of the humerus. Below it is a concave surface (Incisura radialis) which articulates with the back of the head of the radius, and below this is a fossa, which receives a tuberosity of the radius. The distal end (Capitulum ulnæ) is small and is produced to a blunt point (Processus styloideus ulnæ). It articulates with the ulnar carpal below, and has a convex facet on its antero-internal aspect for the radius. [Illustration: FIG. 132.—SKELETON OF DISTAL PART OF THORACIC LIMB OF DOG, EXTERNAL VIEW. _4_, Radial + intermediate carpal bone; _5_, ulnar carpal; _6_, accessory carpal; _7_, second carpal; _8_, third carpal; _9_, fourth carpal; _10–14_, first to fifth metacarpals; _15_, first phalanx; _16_, second phalanx; _17_, third phalanx. (After Leisering’s Atlas.) ]