154 



OSTEOLOGY. 



verted, 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 pubes. In the male it is large, of an oval form, its longest diameter 

 being obliquely from above downwards ; in the female, smaller, and more trian- 

 gular. 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 forwards, inwards, and downwards. 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, inclosed 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 semi-transparent, and composed entirely of compact tissue. 



Development (fig. 99). By eight centres ; three primary one for the ilium, ote 



_ 



.By <S 



Fig. 99. Plan of the Development of the Os Innominatum. 



3 Jcrima.rylJl'vutm/.Jjic/iiunt, K fu/ocfj 



5. Secondary 



xf ^. _ 



TAf S Primary centrr-i finite throuyji. ~Y37iaft4<l fiUce.jaHout fl 

 HpijjJiyset atTveur about vat *r(y f Of unite dboitt 26 ~ yta.r 



for the ischium, and one for the pubes ; and Jive secondary one for the crest of the 

 ilium its whole length, one for the anterior inferior spinous process (said to occur 

 more frequently in the male than in 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, 

 immediately above the sciatic notch, at about the same period that the develop- 

 ment of the vertebrae commences. Secondly, in the body of the ischium, at about 

 the third month of foetal life. Thirdly, in the body of the pubes, 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 



