THE OS INNOMINATUM. 177 



The surface is bounded externally by a rough eminence, the ilio-pectineal, 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, 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, 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 Avhich 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 attachment 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 os pubis is thin and flattened. It 

 passes outward and downward, becoming narroAver as it descends, and joins with 

 the ramus of the ischium. Its anterior surface is rough, for the attachment 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 outward. The 

 posterior surface is smooth, and gives attachment to the Obturator internus. and, 

 close to the inner margin, to the Compressor urethra?. 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 urethra. The outer border 

 is thin and sharp, forms part of the circumference of the obturator foramen, 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 articulation. It 

 presents below a deep notch, the eotijloid notch, which is continuous with a circular 

 depression, the fossa acetabuli, at the bottom of the cavity: this depression is per- 

 forated 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. The 

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

 band which passes across it. Through this foramen the nutrient vessels and 

 nerves enter the joint. 



The obturator or thyroid foramen is a large aperture situated between the 

 ischium and os pubis. In the male it is large, of an oval form, its longest 

 diameter being obliquely from before backward ; in the female it is smaller and 

 more triangular. It is bounded by a thin, uneven margin, to which a strong 

 membrane is attached, and presents, anteriorly, a deep groove which runs from 

 the pelvis obliquely inward and downward. This groove is converted into a 

 foramen by the obturator membrane, and transmits the obturator vessels and 

 nerve 



Structure. This bone consists of much cancellous tissue, especially where it is 

 thick, enclosed between two layers of dense, compact tissue. In the thinner 

 parts of the bone, as at the bottom of the acetabulum and centre of the iliac 

 fossa, it is usually semitransparent, and composed entirely of compact tissue. 



Development (Fig. 122). By eight centres: three primary one for the ilium, 

 one for the ischium, and one for the os pubis ; and five secondary one for the 

 crest of the ilium, one for the anterior inferior spinous process (said to occur more 

 12 



