238 



OSTEOLOGY 





in the superior ramus of the pubis, between the fourth and fifth months. At birth, the three 

 primary centers are quite separate, the crest, the bottom of the acetabulum, the ischial tuberosity, 

 and the inferior rami of the ischium and pubis being still cartilaginous. By the seventh or eighth 

 year, the inferior rami of the pubis and ischium are almost completely united by bone. At out 

 the thirteenth or fourteenth year, the three primary centers have extended their growth into the 

 bottom of the acetabulum, 1 and are there separated from each other by a Y-shaped portion of 

 cartilage, which now presents traces of ossification, often by two or more centers. One of these, 

 the os acetabuli, appears about the age of twelve, between the ilium and 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 pubis and 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 join with the rest of the bone between the twentieth and twenty-fifth years. Separate 

 centers are frequently found for the pubic tubercle and the ischial spine, and for the crest and 

 angle of the pubis. 



Articulations. The hip bone articulates with its fellow of the opposite side, and with the 

 sacrum and femur. 



By eight centers \ ^ hree P rim ^ r V ( Ilium - l^hium, and Pvbis) 

 | Five secondary 



Cv 



FIG. 237. Plan of ossification of the hip bone. The three primary centers unite through a Y-shaped piece about 

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



The Pelvis. 



The pelvis, so called from its resemblance to a basin, is a bony ring, interposed 

 between the movable vertebrae of the vertebral column which it supports, and the 

 lower limbs upon which it rests; it is stronger and more massively constructed 

 than the wall of the cranial or thoracic cavities, and is composed of four bones: 

 the two hip bones laterally and in front and the sacrum and coccyx behind. 



The pelvis is divided by an oblique plane passing through the prominence of 

 the sacrum, the arcuate and pectineal lines, and the upper margin of the symphysis 

 pubis, into the greater and the lesser pelvis. The circumference of this plane is 

 termed the linea terminalis or pelvic brim. 



The Greater or False Pelvis (pelvis major). The greater pelvis is the expanded 

 portion of the cavity situated above and in front of the pelvic brim. It is bounded 

 on either side by the ilium; in front it is incomplete, presenting a wide interval 

 between the anterior borders of the ilia, which is filled up in the fresh state by 



