THE SKELETAL SYSTEM 33 



THE POSTERIOR APPENDAGES 



The two coxal hones (os coxae), sometimes called the in- 

 nominate hones, 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 puhic symphysis. The lateral surface of each bone 

 bears a deep pit, the aeetahulum,, for the reception of the 

 head of the thigh bone (femur). Unlike the pectoral 

 girdle the direct attachment of the pelvic girdle to the back- 

 bone 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 puhis, which radi- 

 ate 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 acetabu- 

 lar wall of the adult is well supported with bone. The 

 acetabulum itself is funnel-shaped. The inner, or con- 

 stricted portion, of this funnel expands medially to form 

 a dorsoventrally elongated pit. 



The ilium (os ilium) is the anterior branch of the tri- 

 radiate coxal bone. It consists of two parts, the hody and 

 the imng. The former is som^ewhat 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 depres- 

 sion along the dorsal border of the ilium, extending back 



