44 THE HORSE IN HEALTH AND DISEASE 



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

 small pastern, the third phalanx or cofhn bone, the proximal pair of 

 sesamoids, and the distal sesamoid or navicular bone. 



The OS coxae 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 portions come together at the floor of the pelvis; their 

 junction point is known as the symphysis. The three bones form- 

 ing the OS coxEe are the ilium, the ischium, and the pubis. They 

 unite at a cavity named the acetabulum, which forms the articular 

 surface for the head of the femur. A direct joint connects the os 

 coxae to the axial skeleton. 



The ilium, the largest of the three bones, is situated on the 

 dorsolateral part of the pelvis and slopes downward and backward. 

 It is triangular in form and composed of a shaft and a wing. The 

 shaft is three sided and joins the other two bones at the acetabu- 

 lum. The wing presents two surfaces, three borders, and three 

 angles. The dorsal surface is concave; the ventral surface is con- 

 vex and carries a facet for articulation with the sacrum. All the 

 borders are concave. The internal angle is thick, forms the 

 highest part of this region, and is known as the angle of the croup. 

 The external angle is large and forms the prominence which, in the 

 living animal, is referred to as the "hook bone," point of the hip, 

 or angle of the haunch. The posterior angle is connected with the 

 shaft. 



The ischium is the most posterior of the bones of the pelvis. 

 It is a flat bone having two surfaces and four borders. Both 

 surfaces are nearly flat. The internal border is thick and rough and 

 is united to its fellow of the opposite side by cartilage. The exter- 

 nal border is concave, as is also the anterior, which forms the pos- 

 terior margin of the obturator foramen. The posterior border is 

 thick and rough. The large, rough tuberosity which projects 

 backward and outward is the "pin bone." 



The pubis is the smallest of the three and is placed at the ante- 

 rior part of the floor of the pelvic cavity. Its dorsal surface is 

 smooth, slightly concave, and supports the bladder. The ventral 

 surface is convex and rough. The external angle of the pubis 

 assists in forming the acetabulum. The posterior border forms 

 the anterior boundary of the obturator foramen. 



In the living animal the term pelvic cavity is applied to the 

 potential cavity which is filled up with the bladder, rectum, and 



