Pelvis

Pelvis, lower part of the trunk of the body, bounded at the front and on either side by the hipbone, and at the back by the sacrum and the coccyx, the lowest part of the spinal column (see Hip; Sacroiliac Joint). The hipbone is composed of three separate bones: the ilium, the ischium or lower part of the hipbone, and the pubis, the central pubic bone that unites with the ischium at either side. In early life, the three bones are separate, then in late teens or early 20s they unite to form a single structure called the innominate bone. At the lower end of the hipbone is a cup-shaped depression called the acetabulum in which the femur, or thighbone, rotates. The pelvis thus acts as a unit in all bodily movements. The weight of the trunk is transferred from the spine through the sacrum and then through the hipbone to the thighbone and the lower extremities. Conversely, all forces acting on the lower limbs are transmitted to the trunk by the same route.

The cavity formed by the pelvic bones contains the lower portion of the intestines and rectum, the urinary bladder, and the internal organs of reproduction. In females, the pelvis is rounder and wider than in males and the bones are lighter, reflecting the greater capacity required for expansion of the uterus in pregnancy and the emergence of the foetus in childbirth.