THE POSTERIOR APPENDAGES
The two coxal bones (os coxae), sometimes called the innominate bones, comprise the pelvic girdle. Each bone is firmly united dorsally to the two anterior sacral vertebrae. Ventrally the two are held together by connective tissue at the pubic symphysis. The lateral surface of each bone bears a deep pit, the acetabulum, for the reception of the head of the thigh bone (femur). Unlike the pectoral girdle the direct attachment of the pelvic girdle to the backbone furnishes a rigid support for the strong and active hind legs of the animal. The coxal bone consists of three fused elements, the ilium, ischium, and pubis, which radiate from the acetabulum. The distal ends of the pubis and ischium are also fused, so that these two bones surround an oval space, the obturator foramen. Though the sutures are completely obliterated in the adult, they can be very easily seen in a very young animal. The cartilaginous boundaries between the three bones in such an animal all lie in the acetabular wall, though the end of the pubis forms only a very small part of this wall. The distal connection of pubis and ischium will be observed at the ventral extremity of the obturator foramen. The acetabular wall of the adult is well supported with bone. The acetabulum itself is funnel-shaped. The inner, or constricted portion, of this funnel expands medially to form a dorsoventrally elongated pit.
The ilium (os ilium) is the anterior branch of the triradiate coxal bone. It consists of two parts, the body and the wing. The former is somewhat flattened laterally, forms the anterior part of the acetabulum, and bears the anterior inferior spine on its lateral surface just in front of the acetabulum. The greater sciatic notch is a depression along the dorsal border of the ilium, extending back
