92 THE SKELETON OF THE HORSE 



medial ligament of the elbow-joint. The lateral border (Margo lateralis) is more 

 strongly curved, but presents no special features. 



The proximal extremity or head ((Extremitas proximalis s. capitulum radii) 

 is flattened from before backward and wide transversely. It presents the hxxmeral 

 articular siirface (Fades articularis humeralis) which corresponds to that of the 

 distal end of the hmnerus; it is crossed by a sagittal ridge, which has a synovial 

 fossa on its posterior part, and ends in front at a prominent lip, the coronoid proc- 

 ess (Processus coronoideus) . Just below the posterior border, and separated by 

 a depression, there are two concave facets (Facies articularis ulnaris) for articula- 

 tion with the ulna, and between these and the interosseous space is a quadrilateral 

 rough area at which the two bones are united by an interosseous ligament. At 

 the medial side of the dorsal surface is the radial or bicipital tuberosity (Tuberositas 

 radii), into which the biceps tendon is inserted. The medial tuberosity (Tuber- 

 ositas proximalis medialis) is continuous with the preceding eminence, and fur- 

 nishes attachment to the short part of the medial ligament. The lateral tuberosity 

 (Tuberositas proximalis lateralis) is more salient; it gives attachment to the lateral 

 ligament and to the common and lateral extensor muscles of the digit. 



The distal extremity is also compressed from before backward. It presents 

 the carpal articular surface (Facies articularis carpea), which consists of three parts. 

 The medial facet is the largest, is quadrilateral, concavo-convex from before back- 

 ward, and articulates with the radial carpal bone; the intermediate one is some- 

 what similar in form but smaller, and articulates with the intermediate carpal bone ; 

 the lateral facet is smaller, is convex, and articulates below ^vith the ulnar carpal 

 and behind with the accessory carpal. The dorsal surface presents three grooves, 

 separated by ridges. The middle one is vertical and gives passage to the tendon 

 of the extensor carpi radialis; the lateral one is similar and contains the tendon of 

 the common digital extensor; the medial one is small and oblique and lodges the 

 tendon of the extensor carpi obliquus. The volar aspect is crossed by a rough 

 ridge, below which are three depressions. On either side is a tuberosity (Tuber- 

 culum ligamenti) to which the collateral ligament is attached. The lateral one is 

 marked by a small vertical groove for the passage of the lateral extensor tendon. 



Development. — The radius ossifies from four centers, viz., one each for the 

 shaft, the two extremities, and the lateral part of the distal end; the last is morpho- 

 logically the distal end of the ulna which has fused with the radius, and the line of 

 fusion is often indicated by a distinct groove on the carpal articular surface. The 

 proximal extremity unites with the shaft at about one and a half years, the distal 

 end at about three and a half years usually. 



THE ULNA 



The ulna of the horse is a reduced long bone situated behind the radius, with 

 which it is partially fused in the adult. 



The shaft (Corpus ulnae) is three-sided and tapers to a point distally. The 

 dorsal surface (Facies dorsalis) is applied to the volar surface of the radius, and be- 

 low the interosseous space the two Isones are fused in the adult. The surface which 

 enters into the formation of the space is smooth and usually presents a small nu- 

 trient foramen, directed upward. Above the space it is rough and is attached to 

 the radius by an interosseous ligament which is usually permanent. The medial 

 surface (Facies medialis) is smooth and slightly concave. The lateral surface 

 (Facies lateralis) is flattened. The medial and lateral borders (Margo medialis, 

 lateralis) are thin and sharp, except at the interosseous space. The volar border 

 (IMargo volaris) is slightly concave in its length and is rounded. The distal end is 

 pointed and is usually a little below the middle of the radius. It is commonly 



