84 



THE SKELETON OF THE HORSE 



the radius. The inferior or distal surface is smaller, convex in front, concave be- 

 hind, and articulates with the third and fourth carpal bones. The internal surface 

 has upper and lower facets for articulation with the radial carpal, and l)etween 

 these it is excavated and rough. The external surface is similar to the preceding 

 and articulates with the ulnar carpal. The anterior or dorsal surface is rough and 

 slightly convex. The posterior or volar surface bears a tuberosity on its lower part. 



The Ulnar Carpal Bone 

 The ulnar carpal ])one (Os carpi ulnare, cuneiform) is the smallest and most 

 irregular l)one of the upper row. The superior or proximal surface is concave, and 

 fits the lower part of the outer facet on the distal end of the radius. The inferior 

 or distal surface is oblique and undulating for articulation with the fourth carpal 

 bone. The internal surface has upper and lower facets for articulation with the 

 intermediate. The anterior or dorsal and external surfaces are continuous, convex, 

 and rough. The posterior or volar surface is oblique, and bears a concave facet 

 for articulation with the accessory carpal bone; below this is a tubercle. 



Fig. 52. — Left Carpal Bones of Horse, with Dip- Fig. 53. — Left Carpal Bones of Horse, -mTH 

 TAL End of Radius and Proximal End of Adjacent Ends of Radius and Metacarpus; 



Metacarpus; Internal View. Extern.\l View. 



Ca, Accessory carpal bone; Cr, radial carpal; Ci, intermediate carpal; Cu, ulnar carpal; Cl-4, first to 



fourth carpals; Mc. II, III, IV, metacarpal bones; 1, groove for tendon of extensor carpi obliquus; 2, groove for 



lateral extensor tendon; 3, groove for tendon of flexor carpi externus; 4, metacarpal tubero.sity. (After Schmaltz, 



Atlas d. Anat. d. Pferdes.) 



The Accessory Carpal Bone 

 The accessory carpal ]>one (Os carj)i accessorium, pisiform) is situated behind 

 the ulnar carpal bone and the outer part of the distal end of the ulna. It is discoid 

 and ])resents for description two surfaces and a circumference. The internal 

 surface is concave and forms the outer wall of the carpal groove. The external 

 surface is convex and rough; a smooth groove for the outer tendon of the flexor 

 carpi externus cross(>s its anterior part obliquely downward and slightly forward. 

 The anterior border bears two facets; the upper one is concave and articulates with 

 the back of the outer facet on the distal end of the radius ; the lower one is convex 

 and articulates with the ulnar carpal bone. The remainder of the circumference is 

 rounded and rough. 



The accessory does not directly bear weight, and may be regarded as a sesamoid bone 

 interposed in the course of the tendons of the middle and external flexors of the carpus, which 

 it enables to act at a meciianical advantage. The posterior l)order furnishes attachment to 

 the transverse carpal ligament, which completes the carpal canal for the flexors of the digit. 



