One important landmark on the ischium is the ischialspine—a projection just inferior to the greater sciaticnotch. The ischial tuberosity is the roughened pro-jection inferior to the ischial spine. This is the bone in your buttock that bears your weight when you sit. The ischium has a projection called the ischial ramus, which continues with the projection inferior ramus of the pubis. Together with the superior ramus of the pubis, the rami enclose an opening called the obtura-tor foramen. In life, this foramen is lined by connec-tive tissue that provides a base for attachment of mus-cles both on the interior and exterior surfaces. The medial surface of the pelvis has a shallow depression called the iliac fossa.
The pelvis is divided into the true (lesser) and the false (greater) pelvis. The true pelvis is the regionbelow an imaginary line that runs from the superior aspect of the sacrum to the superior margin of the pubic symphysis. The upper bony edge of the true pelvis is the pelvic brim and the opening is the pelvic inlet. When an obstetrician says that the headof the baby is fixed, it indicates that the head has en-tered the pelvic inlet.
The pelvic outlet is the opening bound by the in-ferior edges of the pelvis. This region is called the perineum in life and is bound by the coccyx, the is-chial tuberosities, and the inferior border of the pu-bic symphysis. Strong perineal muscles support the organs in the pelvic cavity.
The male and female pelvis differs in shape and size. In females, the pelvis is lighter and smoother, with less prominent markings. The entire pelvis is low and broad. To facilitate childbearing, both the pelvic inlet and outlet are larger and wider in females. The arch made by the inferior rami of the pubis (pubic arch) is wider and the sacrum and coccyx are less curved, widening the pelvic outlet. Hormones secreted at pregnancy soften and loosen the ligaments and carti-lage in the pelvis, enabling the pelvis to widen fur-ther, if necessary, at delivery.
In females, the acetabulum is small and faces an-teriorly compared with that of males, where it is larger and faces laterally. This is partly responsible for the difference in gait between men and women. The shape of the obturator foramen is also different, being oval in females and round in males.