PELVIC APERTURES. 87 



antero-inferior part of the pelvis ; it consists of a body and a 

 ramus. 



The body reaches from the acetabulum to the median sym- 

 physis ; its superior surface is concave to receive the urinary 

 bladder, and its anterior surface is convex, and transversely 

 crossed by a deep groove from the acetabulum, which contains 

 the pubio-femoral ligament. The crest is the roughened anterior 

 border, which terminates at the symphysis. The posterior border 

 extends to the ramus, and forms the anterior margin of the 

 obturator foramen ; at its junction with the ilium it is crossed 

 by a groove for the obturator vessels. The outer extremity 

 which joins the ilium and the ischium in the acetabulum, is 

 excavated to form the cotyloid notch. The ramus is the flattened 

 portion which projects posteriorly, joins the ramus of the ischium, 

 forming part of the boundary of the obturator foramen, and 

 with its fellow the symphysis pubis internally. 



ACETABULUM. 

 (FiG. 20. 4.) 



The cotyloid cavity or aceta.bv.lurn is situated on the extero- 

 lateral aspect of the pelvis, where the three segments of the os 

 innominatum unite. It receives the head of the thigh-bone, and 

 is one of the deepest articular depressions in the body, though 

 relatively it is much less capacious in the horse and in quadru- 

 peds generally than in man. It is circular in outline, and, 

 except at its inferior median part, is surrounded by a prominent 

 lip of bone, which becomes thin and slightly roughened at its 

 free surface for muscular and ligamentous attachment ; its inferior 

 border presents a large notch, the cotyloid notch, continued 

 half-way across the articulation as the fundus acetdbuli, to give 

 attachment to the round ligament (ligamentum teres), which 

 mainly occupies it. Although the cotyloid cavity is formed by 

 the three segments of the os innominatum, the ilium and the 

 ischium are generally the only parts which form its articular 

 surface, the pubis in most cases merely helping to form the 

 roughened notch and fundus. 



PELVIC APERTURES. 



The obturator or thyroid foramen, or foramen ovale, the 

 largest foramen in the body, exists in each os innominatum, and 



