THE BONES OF THE LOWER LIMB 227 



which is formed mainly by the ischium, is depressed, and lodges 

 the Haversian gland of the hip-joint. 



The obturator or thyroid foramen hes below, and internal to, the 

 acetabulum, its boundaries being formed by the ischium and os 

 pubis. Its long diameter is directed downwards and outwards, 

 and it is oval in the male, but triangular, with rounded angles, in 

 the female. Its circumference is sharp for the obturator membrane, 

 which closes the opening, except opposite the obturator groove 

 superiorly, where it converts that groove into the obturator canal. 



The great and small sciatic notches are situated on the posterior 

 border of the bone, and are separated from each other by the spine 

 of the ischium. The great notch is formed partly by the ihirni, 

 and partly by the ischium; and the small notch hes between the 

 ischial spine and tuber ischii. 



In the recent state these notches are converted into foramina by 

 the great and small sacro-sciatic hgaments. For the structures which 

 pass through these foramina, see the description of the gluteal region. 



Appears about the 8th Week ... 

 (intra-uterine) 



Appears about the 5th Month ,»=«!■> ^ '• 



(intra-uterine) Vv.y^^<?^''^^'^PP**'^ about the 3rd Month 



(intia-uterioe) 



Fig. 133. — Ossification of the Os Innominatum. 



The OS innominatum is pierced by a great number of nutrient 

 foramina for arteries, the chief of which are situated as follows: 

 along the inner aspect of the crest for branches of the deep cir- 

 cumflex ihac; in the iliac fossa near the auricular surface, where 

 there are one or two for branches of the ilio-lumbar; on the external 

 surface of the ihum, and around the margin of the acetabulum 

 for branches of the gluteal; between the acetabulum and tuber 

 ischn for branches of the obturator; on the ilio-pectineal eminence 

 for branches of the deep circumflex ihac; and over the body of the 

 OS pubis for branches of the obturator, and of the pubic branches 

 of the common femoral. 



Articulations.— Pos^^r/or/y with the sacrum, externally with the 

 femur, and tnternally with its fellow. 



Ossification.— The os innominatum is ossified in cartilage from 3 primary 

 and 9 secondary centres. The primary centres are ihac, ischial, and pubic. 

 I he iliac centre appears in the Sth week ; the ischial centre in the ^rd month ; 

 and the pubic centre in the sth month in the superior pubic ramus. 



The ischial and inferior pubic rami join about the &th year. 



