THE PELVIS 



175 



are of considerable size, but are surrounded by relatively wide tracts of cartilage; ossification 

 has, however, extended into the margin of the acetabulum. In the eighth year the rami of the 

 pubis and ischium become united by bone, and in the twelfth year the triradiate cartilage which 

 separates the three segments of the bone in the acetabulum begins to ossify from several centres. 

 Of these, one is more constant than the others and is known as the acetabular nucleus. The 

 triangular piece of bone to which it gives rise is regarded as the representative of the cotyloid 

 or acetabular bone, constantly present in a few mammals. It is situated at the medial part of 

 the acetabulum and is of such a size as to exclude entirely the pubis from the cavity. With 

 this bone, however, it eventually fuses, and afterward becomes joined with the ilium and 



Fig. 210. — Coxal or Hip-bone (Inner Surface) at the Eighth Year. 



Ilium 



_v Pubis 



Ischium 



ischium, so that by the eighteenth or twentieth year the several parts of the acetabulum have 

 become united. In the fifteenth year other centres appear in the cartilage of the crest of the 

 ilium, the anterior inferior iliac spine, the tuberosity of the ischium, and the pubic pecten. The 

 epiphyses fuse with the main bone about the twentieth year. The fibrous tissue connected 

 with the tubercle of the pubis represents the epipubic bones of marsupials. 



THE PELVIS 



The pelvis (figs. 211, 212, 213, 214) is composed of four bones: the two coxal 

 or hip-bones, the sacrum, and the coccyx. The hip-bones form the lateral and 

 anterior boundaries, meeting each other in front to form the pubic symphysis 

 [symphysis ossium pubis]; posteriorly they are separated by the sacrum. The 

 interior of the pelvis is divided into the major and minor pelvic cavity. 



The major (or false) pelvis is that part of the cavity which lies above the ter- 

 minal (iliopectineal) lines and between the iliac fossae. This part belongs really to 

 the abdomen, and is in relation with the hypogastric and iliac regions. 



The minor (or true) pelvis is situated below the terminal (ilio-pectineal) hnes. 

 The upper circumference, known as the superior aperture (inlet or brim) of the 

 pelvis, is bounded anteriorly by the tubercle and pecten of the pubis on each side, 

 posteriorly by the anterior margin of the base of the sacrum, and laterally by the 

 terminal Hnes. The inlet in normal pelves is heart-shaped, being obtusely pointed 

 in front; posteriorly it is encroached upon by the promontory of the sacrum. It 

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

 diameter [conjugata], is measured from the sacro-vertebral angle to the symphysis. 

 The transverse diameter represents the greatest width of the pelvic cavity. The 

 oblique diameter is measured from the sacro-iUac synchondrosis of one side to the 

 ilio-pectineal eminence of the other. 



The cavity of the minor (true) pelvis is bounded in front by the pubes, behind 

 by the sacrum and coccyx, and laterally by a smooth wall of bone formed in part 

 by the ilium and in part by the ischium. The cavity is shallow in front, where it 

 is formed by the pubes, and is deepest posterior^. 



