178 



THE SKELETON. 



, 



frequently in the male than the female), one for the tuberositj 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 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 mont'h of foetal life; 

 thirdly," in the body of the os pubis, between the fourth and fifth months. At 

 birth the three primary centres are (juite 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 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. 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 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 between 

 the twentieth and twenty-fifth years. Separate centres are frequently found for 

 the pubic and ischial spines. 



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



f Three primary (Ilium, Ischium. and Os Pubis). 

 By eight centres j Fiv * 



S. Symphysis pubis. 



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

 Epiphyses appear about puberty, and unite about 25th year. 



FIG. 122. Plan of the development of the os innominatum. 



Attachment of Muscles. To the ilium, sixteen. To the outer lip of the crest, 

 the Tensor vaginae femoris, Obliquus externus abdominis, and Latissimus dorsi ; 

 to the internal lip, the Iliacus, Transversalis, Quadratus lumborum, and Erector 

 spinae ; to the interspace between the lips, the Obliquus internus. To the outer 

 surface of the ilium, the Gluteus maximus, Gluteus medius, Gluteus minimus, 

 reflected tendon of the Rectus ; to the upper part of the great sacro-sciatic notch, a 

 portion of the Pyriformis ; to the internal surface, the Iliacus : to that portion of 



