THE PELVIS 



145 



In the fourth month a nucleus is seen in the puliic portion of the cartilage. At 

 l>irtli these three nuclei are of considerable size, but surrounded by relatively wide 

 tracts of cartilage. At the twelfth year the triradiate cartilage which separates the 

 three segments of the innominate bone at the acetabulum begins to ossify by several 

 centres, which unite with the surrounding elements causing their consolidation from 

 the eighteenth to the twentieth year. One of these nuclei is larger than the others 

 and is the acL'tal)ular nucleus. The segment it forms is regarded by some mori:>holo- 

 gists as the representative of the cotyloid or acetabuldr bone constantly present in a 

 few mammals, and is of sufficient size to exclude the pubes from the cotyloid cavity. 

 During the eighth year the rami of the ischium and pulses coalesce. About the 

 lifteenth year two secondary nuclei appear in the iliac cartilage to form the crest 

 and the anterior inferior spine. An accessory nucleus appears for the ischial tuber- 

 osity, and subsequently one for the pubic crest. These fuse with the main bone 

 about the twentieth year. The fibrous tissue connected with the jjubic spine repre- 

 sents the epipubic bone of marsupial mammals. 



THi: PELVIS 



The pelvis is composed of four bones: the two hip-bones, the sacrum, and the 

 coccyx. The hip-bones form the lateral and anterior boundaries, meeting each 

 other to form the pubic symphysis; posteriorly they are separated by the sacrum. 

 The hollow of the pelvis is divided into the false and true jielvic cavity. 



Fig. 157.— The Pelvis (Male) 



The false pelvis is tliat part of the cavity wliich lies above the ilio-pectineal 

 lines; this part is in relation with the hypogastric and inguinal regions. 



The true pelvis is situated below the ilio-pectineal lines. The upper circum- 

 ference, called also the inlet of the pelvis, is bounded anteriorly by the spine and 

 crest of the pubes, posteriorly by the l^ase of the sacrum, and laterally by the ilio- 

 ])ectineal lines. The inlet in normal pelves is cordate, being obtusely pointed in 

 front; posteriorly it is eneroached upon by the promontory of the sacrum. It has 

 three principal diameters ; of these, the antero-posterior, called the conjugate 

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