42 THE HORSE IN HEALTH AND DISEASE 



animal. The upper and lower surfaces present a number of facets 

 for articulation with the neighboring structures. 



The metacarpus is composed of the cannon and two splint 

 bones. In most mammals five metacarpal bones are present, 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 

 upper extremity presents several flat facets for the corresponding 

 surface of the carpus, while the lower end has two condyles with a 

 prominent central ridge for articulation with the pastern and two 

 sesamoids. The splints are incompletely developed long bones 

 and are three sided. The upper extremity of each is enlarged and 

 articulates with the adjacent bones. The distal extremity is a 

 small nodule which can be easily felt in the living animal by run- 

 ning the hand downward along the cannon, pressing gently with 

 the fingers. This prominence is referred to as "the button of the 

 splint" and is situated from two-thirds to three-fourths the way 

 down the region. 



The first phalanx, or large pastern, is a long bone situated be- 

 tween the cannon and the small pastern in a direction obliquely 

 downward and forward. It forms an angle of 50 to 55 degrees 

 with the horizontal plane in well-formed limbs. The degree of 

 inclination varies considerably in different horses. When the 

 slope is excessive, undue weight is thrown upon the tendons and 

 ligaments at the back of the region; on the other hand, if the bone 

 is too straight, nearly the entire weight of the animal is thrown 

 upon the bones of the foot. The first phalanx presents for de- 

 scription a shaft and two extremities (see Fig. 60). The anterior 

 surface of the shaft is convex, the posterior surface is flattened and 

 roughened. The upper end consists of two shallow cavities for 

 articulation with the large metacarpal bone. The distal end is 

 much smaller and articulates with the small pastern. 



The second phalanx, or small pastern, has a very short shaft. 



