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 humeral 
articular surface (Facies articularis humeralis) which corresponds to that of the 
distal end of the humerus; 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 
radi), to 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 with 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 ulne) 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 bones 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 
(Margo 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 
