278 THE SKELETON. 



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 urethrse. 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 anterior layer of the 

 triangular ligament of the perinaeum. 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 a fibro- 

 cartilaginous structure which contracts its orifice and deepens the surface for 

 articulation. It presents, on its inner side, a deep notch, the cotyloid notch, which 

 is continuous with a circular depression, the fossa acetabuli, at the bottom of the 

 cavity : this depression is 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. 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 above downward ; 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 at its upper and outer part a deep groove which runs 

 from the pelvis obliquely forward, 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. 209). 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 

 frequently in the male than the female), one for the tuberosity of the ischium, one 

 for the symphysis pubis (more frequent in the female than the male), and one for 

 the Y-shaped piece at the bottom of the acetabulum. These various centres appear 

 in the following order : First, in the ilium, at the lower part of the bone, imme- 

 diately above the sciatic notch, at about the eighth or ninth week ; secondly, in the 

 body of the ischium, at about the third month of foetal life ; thirdly, in the body of 

 the os pubis, between the fourth and fifth months. At birth the three primary 

 centres are quite separate, the crest, the bottom of the acetabulum, and the rami 

 of the ischium and pubes being still cartilaginous. At about the seventh or eighth 

 year the rami of the os pubis and ischium are almost completely ossified. About 

 the thirteenth or fourteenth year the three divisions of the bone have extended 

 their growth into the bottom of the acetabulum, being separated from each other 

 by a Y-shaped portion of cartilage, which now presents traces of ossification, often 

 by two or more centres. The ilium and ischium then become joined, and lastly 

 the os pubis, through the intervention of this Y-shaped portion. At about the age 

 of puberty ossification takes place in each of the remaining portions, and they 

 become joined to the rest of the bone about the twenty-fifth year. 



Articulations. With its fellow of the opposite side, the sacrum, and femur. 



