THE SKELETAL SYSTEM 43 



with the distal end of the humerus, the inner of which is the 

 larger. Posteriorly are two facets for articulation with the ulna. 

 In front at the inner side is the large bicipital tuberosity, to which 

 the lower tendon of the biceps muscle is inserted. The distal 

 extremity is composed of three facets for articulation with the 

 proximal row of carpal bones. 



The ulna is a reduced long bone occupying a position behind 

 the radius and distal part of the humerus. It presents a shaft 

 and two extremities. The shaft is three sided and tapers toward 

 the lower extremity. The anterior surface is attached to the 

 rough area noticeable on the corresponding part of the radius. 

 The internal surface is concave and smooth, while the outer is 

 flattened. The proximal extremity forms the bulk of the bone 

 and projects upward and backward. Its summit is termed the 

 olecranon process and corresponds to the "point of the elbow" 

 in man. The anterior border bears the semilunar notch. 



The bones of the carpus are either seven or eight in number and 

 arranged in two rows. Named from within outward the upper 

 row is composed of the radial, intermediate, ulnar, and accessory 

 carpal bones; the lower row of the first, second, third, and fourth. 

 The first carpal is often absent or the size of a pea. The carpus, 

 as a whole, has four surfaces. The anterior surface is convex 

 and lies further back than the distal end of the radius, thus afford- 

 ing protection to the joint when the animal falls on the knee. 

 The posterior surface is very rough in the macerated state, but is 

 leveled by the thick posterior ligament in the living animal. The 

 upper and lower surfaces present a number of facets for articula- 

 tion with the neighboring structures. 



The metacarpus is composed of the cannon and two splint 

 bones. Most mammals have five metacarpal bones, but the 

 horse has only the second, third, and fourth. Number three, 

 the large metacarpal or cannon bone, is the only one fully devel- 

 oped. The cannon bone is one of the strongest bones of the body, 

 and is situated between the carpus above- and the large pastern 

 below. It presents for description a shaft and two extremities. 

 The anterior surface of the shaft is smooth and rounded trans- 

 versely. The posterior surface is rather flat and with the splint 

 bones forms a shallow channel for the suspensory ligament. In 

 the adult sometimes the small bones are firmly united to the 

 cannon,' causing the bony enlargement known as "splint." The 



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