THE OS INNOMINATUM 213 



sharp margin, which forms part of the circumference of the obturator foramen and 

 affords attachment to the obturator membrane. 



The ascending or superior ramus of the pub ; s (ramus superior ossis pubis) extends 

 from the body to the point of junction of the os pubis with the ilium, and forms the 

 upper part of the circumference of the obturator foramen. It presents for examina- 

 tion a superior, inferior, and posterior surface, and an outer extremity. The supe- 

 rior surface presents a continuation of the iliopectineal line, already mentioned as 

 commencing at the pubic spine. In front of this ridge the surface of bone is 

 triangular in form, wider externally than internally, smooth, and is covered by 

 the Pectineus muscle. The surface is bounded externally by a rough eminence, 

 the iliopectineal eminence (cwincntia iliopectinea), which serves to indicate the 

 point of junction of the ilium and os pubis, and gives attachment to the Psoas 

 parvus, when this muscle is present. The triangular surface is bounded below 

 by a prominent ridge, the obturator crest (crista obturatoria), which extends from 

 the cotyloid notch to the spine of the os pubis. The inferior surface forms the 

 upper boundary of the obturator foramen, and presents externally a broad and 

 deep oblique groove, the obturator groove (sulcus obturatorius), for the passage 

 of the obturator vessels and nerve; and internally a sharp margin which forms 

 part of the circumference of the obturator foramen, and to which the obturator 

 membrane is attached. The posterior surface forms part of the anterior boundary 

 of the true pelvis. It is smooth, convex from above downward, and affords 

 origin to some fibres of the Obturator internus. The outer extremity, the thickest 

 part of the ramus, forms one-fifth of the cavity of the acetabulum. 



The descending or inferior ramus of the pubis (ramus inferior ossis pubis) is thin 

 and flattened. It passes outward and downward, becoming narrower as it descends, 

 and joins with the ramus of the ischium. Its anterior surface is rough, for the 

 origin of muscles the Gracilis along its inner border; a portion of the Obturator 

 externus where it enters into the formation of the foramen of that name; and 

 between these two muscles the Adductores brevis and magnus from within out- 

 ward. The posterior surface is smooth, and gives origin to the Obturator internus, 

 and, close to the inner margin, to the Compressor urethrae. The inner border is 

 thick, rough, and everted, especially in females. It presents two ridges, separated 

 by an intervening space. The ridges extend downward, and are continuous 

 with similar ridges on the ascending ramus of the ischium; to the external one 

 is attached the deep layer of the superficial perineal fascia, and to the internal 

 one the superficial layer of the triangular ligament of the perineum. The outer 

 border is thin and sharp, forms part of the circumference of the obturator fora- 

 men, and gives attachment to the obturator membrane. 



The Cotyloid Cavity, or Acetabulum, is a deep, cup-shaped, hemispherical 

 depression, directed downward, outward, and forward; formed internally by the 

 os pubis, above by the ilium, behind and below by the ischium, a little less than 

 two-fifths being formed by the ilium, a little more than two-fifths by the ischium, 

 and the remaining fifth by the pubic bone. It is bounded by a prominent uneven 

 rim, which is thick and strong above, and serves for the attachment of the 

 cotyloid ligament, which contracts its orifice and deepens the surface for articula- 

 tion. It presents below a deep notch, the cotyloid notch (incisura acetabuli), 

 which is continuous with a circular depression, the fossa of the acetabulum (fossa 

 acetabuli), at the bottom of the cavity; this depressiorfTs perforated by numerous 

 apertures, lodges a mass of fat, and its margins, as well as those of the notch, 

 serve for the attachment of the ligamentum teres. In front, above and behind 

 the fossa acetabuli, is a concave rim of bone (fades lunata). The cotyloid notch 

 is converted, in the natural state, into a foramen by a dense ligamentous band, 

 the transverse ligament, which passes across it. Through this foramen the 

 nutrient vessels and nerves of the joint pass. 



