THE SKELETAL SYSTEM 45 



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 attach- 

 ment to a lateral cartilage. 



The lateral cartilages are two rhombic-shaped masses of carti- 

 lage 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 flexi- 

 ble. When they undergo ossification, "side-bone" is the result 

 (see Fig. 73). 



The sesamoids 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 of 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 patella or 

 stifle, the tibia and fibula or leg, the tarsus or hock, the metatarsus 

 or cannon, the first phalanx or large pastern, the second phalanx 

 or small pastern, the third phalanx or coffin bone, the proximal 

 pair of sesamoids, and the distal sesamoid or navicular bone. 



The os coxse, or hip, is a large, flat bone which forms part of the 

 lateral wall and the entire ventral wall of the pelvic cavity. The 

 right and left bones come together at the floor of the pelvis; 

 their junction point is known as the symphysis. In the fetus 

 three bones join to form each os coxa, viz., the ilium, the is- 

 chium, and the pubis. These unite at a cavity named the ace- 

 tabulum, which forms a socket for the head of the femur. A direct 

 joint connects the os coxae to the sacrum of the axial skeleton. 



The ilium, the largest of the three bones of the os coxaa, slopes 

 downward and backward. It is triangular in form and divided 

 into a shaft and a wing. The shaft is three sided and joins the 

 other two bones at the acetabulum. The wing presents two sur- 

 faces, three borders, and three angles. The dorsal surface is 

 concave; the ventral surface is convex and carries a facet for 

 articulation with the sacrum. All the borders are concave. 



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