The lower circumference of the pelvis is very irregular; the space enclosed by it is named the pelvic outlet (inferior aperture), 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 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; laterally, 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.