82 



THE SKELETON OF THE HORSE 



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

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

 ward, and articulates with the radial carpal bone (or scaphoid) ; the middle one is 

 somewhat similar in form but smaller, and articulates with the intermediate carpal 

 bone (or semilunar); the outer facet is smaller, is convex, and articulates below 

 with the ulnar carpal (or cuneiform) and Ijehind with the accessory carpal (or pisi- 

 form). The anterior surface presents three grooves, separated by ridges. The 

 middle one is vertical and gives passage to the tendon of the extensor carpi radialis; 

 the outer one is similar and contains the tendon of the anterior extensor of the 

 digit; the inner one is small and oblique and lodges the tendon of the extensor carpi 

 obliquus. The posterior aspect is crossed l)y a rough ridge, below which are three 

 depressions. On either side is a tuberosity (Tuberculum ligamenti) to which the 

 lateral ligament is attached. The outer 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 outer part 

 of the distal end ; the last is morphologically 

 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. 



Processus 



(INCOtlCCUS 



Semilunar notch 



Bicipital 

 tuberosity 



Interosseou.'i 

 space 



Fig. 50. — Upper Half of R.\diu.s and Ulna of 

 Horse, Internal View. (After Schmaltz, 

 Atlas d. Anat. d. Pferdes.) 



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 below. The anterior 

 surface (Facies dorsalis) is applied to the pos- 

 terior surface of the radius, and below 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 nutrient foramen, directed 

 upward. Above the space it is rough and is 

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

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

 surface (Facies lateralis) is flattened. The internal and external borders are thin 

 and sharp, except at the interosseous space. The posterior border is slightly 

 concave in its length and is rounded. The lower end is pointed and is usually a 

 little below the middle of the radius. It is commonly continued 1)3' a fibrous cord 

 to the distal external tuberosity of the radius, but this band may be replaced in 

 part or entirely by bone. 



The proximal extremity is the major part of the bone. It projects upward 

 and somewhat Ijackward 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 alwve. The anterior 

 border bears on its middle a pointed projection, the processus anconaeus or 

 "beak," which overhangs the semilunar notch or sigmoid cavity (Incisura semilu- 

 naris). The latter is triangular in outline, concave from above downward, and 



