THE PELVIS. 131 



of the hip-joint. The obturator foramen is a large hole, 

 bounded above by the bodies of the ischium and pubes, 

 below by the rami of the pubes and ischium, and poste- 

 riorly by the anterior border of the ischium. It is tri- 

 angular in form, smaller in the female than in the male, 

 and gives attachment to the thyroid or obturator mem- 

 brane. Its upper border presents a groove for the 

 obturator vessels and nerve. 



The os innominatum develops by three centres, one 

 for the ilium, one for the ischium, and one for the pubes; 

 in addition, there are four or five secondary centres. 

 The three segments unite in the condyloid cavity, of 

 which the ilium forms the upper two-fifths, the ischium 

 the posterior and lower two-fifths, and the pubes the 

 anterior and lower one-fifth. The rami of the ischium 

 and pubes unite about the seventh year. 



THE PELVIS. 



The pelvis is formed by the two ossa innominate, 

 sacrum, and coccyx. It is a ring of bone, through which 

 the weight of the trunk, head, and upper extremities is 

 transmitted to the lower extremities. It consists of two 

 parts, an upper, expanded portion, the false pelvis ; 

 an inferior, cylindrical part, the true pelvis. The false 

 pelvis is deficient anteriorly ; laterally and posteriorly 

 it is formed by the expanded ilia and lumbar vertebrae. 

 It is separated from the true pelvis by the ilio-pectineal 

 ridge or line, which begins at the spine of the pubes and 

 terminates at the promontory of the sacrum. 



The inlet to the true pelvis, called the superior strait, 

 is formed by the ilio-pectineal ridge. It is elliptical in 

 outline, somewhat encroached upon posteriorly by the 

 promontory of the sacrum. The true pelvis is a curved 

 cylinder of bone, longer posteriorly than anteriorly, and 



