THE SKELETAL SYSTEM 43 



The bone presents four surfaces. The proximal surface has two 

 shallow cavities for articulation with the distal extremity of the 

 first phalanx. The distal surface is molded for articulation with 

 the coffin and navicular bones. The anterior and posterior sur- 

 faces are convex and slightly roughened. 



The third phalanx, or coflEln bone, is entirely concealed in the 

 hoof, to which it closely conforms in shape. It presents three 

 surfaces and two angles or wings. The articular surface has two 

 shallow cavities for articulation with the small pastern, behind 

 which is a flat area for the navicular bone. In front is seen the 

 extensor process for attachment of the tendon of the extensor 

 muscle. The anterior or wall surface slopes downward and for- 

 ward and corresponds to the inner surface of the wall of the hoof; 

 it has a rough, porous appearance due to the large number of for- 

 amina which it contains. The inferior surface is clearly divided 

 into two parts by the semilunar crest which gives insertion to the 

 deep flexor tendon. The anterior portion is vaulted, crescent 

 shaped, and is termed "the sole surface," as it corresponds to the 

 horny outer sole. The posterior portion is less extensive and is 

 named "the tendinous surface." The angles or wings project back 

 on either side. The upper edge of each wing gives attachment to a 

 lateral cartilage. 



The lateral cartilages are two rhombic-shaped masses of 

 cartilage which curve backward toward each other at the heel. 

 The abaxial surface of each is convex; the axial is concave. These 

 cartilages extend above the horny hoof wall and can be easily felt 

 in the living animal. In their natural state they should be flexible. 

 When they undergo ossification, "side-bone" is the result (see 

 Fig. 60). 



The sesamoid bones are two small bones which articulate with 

 the distal end of the cannon. In their fresh state they are firmly 

 bound together by a disk-shaped mass of cartilage over which the 

 flexor tendons play. 



The navicular bone, or distal sesamoid, is situated back of the 

 coffin bone and lower end of the small pastern. It articulates 

 with both 6f these phalanges. The deep flexor tendon plays over 

 its posterior surface. 



B. The bones of the pelvic or hind limb, named from above down- 

 ward, are the os coxae or hip, the femur or thigh, the tibia and fibula 

 or leg, the patella or stifle, the tarsus or hock, the metatarsus or 



