240 



OSTEOLOGY 





The Lower Circumference. The lower circumference of the pelvis is very irregu ar; 

 the space enclosed by it is named the inferior aperture or outlet (apertura pelvis 

 [minoris] inferior) (Fig. 239), and is bounded behind by the point of the coccyx, 

 and laterally by the ischial tuberosities. These eminences are separated by three 

 notches: one in front, the pubic arch, formed by the convergence of the inferior 



FIG. 239. Diameters of inferior aperture of lesser pelvis (female). 



rami of the ischium and pubis on either side. The other notches, one on either 

 side, are formed by the sacrum and coccyx behind, the ischium in front, and 

 the ilium above; they are called the sciatic notches; in the natural state they are 

 converted into foramina by the sacrotuberous and sacrospinous ligaments. When 

 the ligaments are in situ, the inferior aperture of the pelvis is lozenge-shaped, 



bounded, in front, by the pubic arcuate ligament 

 and the inferior rami of the pubes and ischia; later- 

 ally, by the ischial tuberosities; and behind, by the 

 sacrotuberous ligaments and the tip of the coccyx. 



The diameters of the outlet of the pelvis are two, 

 antero-posterior and transverse. The antero-posterior 

 diameter extends from the tip of the coccyx to the 

 lower part of the pubic symphysis; its measurement 

 is from 90 to 115 mm. in the female. It varies with 

 the length of the coccyx, and is capable of increase 

 or diminution, on account of the mobility of that 

 bone. The transverse diameter, measured between 

 the posterior parts of the ischial tuberosities, is about 

 115 mm. in the female. 1 



Axes (Pig. 240). A line at right angles to the plane 

 of the superior aperture at its center would, if prolonged, 

 pass through the umbilicus above and the middle 

 of the coccyx below ; the axis of the superior aperture 

 is therefore directed downward and backward. The 



Fio. 240. Median sagittal section of ...... , , , 



pelvis. axis ot the inferior aperture, produced upward, would 



touch the base of the sacrum, and is also directed 



downward, and slightly backward. The axis of the cavity i. e., an axis at right 

 angles to a series of planes between those of the superior and inferior apertures 



1 The measurements of the pelvis given above are fairly accurate, but different figures are given by various authors 

 no doubt due mainly to differences in the physique and stature of the population from whom the measurements have 

 been taken. 



