DEVELOPMENT OF OS INNOMINATUM. 233 



brane 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 of the Os Innominalum. This bone consists of much cancellous tis- 

 sue, 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. 166). By eight centres : three primary one for the ilium, 

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



Fig. 166. Plan of the Development of the Os Innominatum. 

 JBy S Centre*. | 



\ S.Hcrontftt ry 



TAe 3 Primary e0ntre* <unitt l&roupZ. "YiSJutfuH /liece 



affJtnr about j,u,1>arty,k unite about 2 5$ year 



crest of the ilium its whole length, one for the anterior inferior spinous process 

 (said to occur more frequently in the male than the female), one for the tuber- 

 osity 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 development of the vertebra} 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 cartilaginous. At about the 

 sixth year, the rami of the pubes 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. The ilium and ischium then become joined, and lastly the pubes, 



