214 



SPECIAL ANATOMY OF THE SKELETON 



The Obturator, or Thyroid Foramen (foramen obturatum), is a large aperture 

 situated between the ischium and os puhis. 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, 

 the obturator groove (sulcus obturator ius), which runs from the pelvis obliquelv 

 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. 169). From 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 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 or more for the Y-shaped piece at the bottom of the acetabulum. These 



From eight centres 



8. Symphysis pubis. 



The three primary centres unite through a \-shaped piece about puberty. 

 Epiphyses appear about puberty, and unite about the twenty-fifth year. 



FIG. 169. Plan of the development of the os innominatum. 



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 eighth or ninth week; secondly, in the body of 

 the ischium, at about the third month of fetal 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, the ischial tuberosity, 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 united by bone. About the twelfth 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. One of these, the os acetabuli, appears about the age of twelve, between the ilium and os 

 pubis, and fuses with them about the age of eighteen. It forms the pubic part of the acetabulum. 

 The ilium and ischium then become joined, and lastly the os pubis to the ischium, through the 



