Bone · Pelvis

Sacrum

Os sacrum

The sacrum is a triangular block of five vertebrae fused into a single bone, wedged between the two hip bones at the back of the pelvis. It is the keystone of the pelvic ring: the whole weight of the spine passes through it, out across the sacroiliac joints and into the legs. Its back surface is roughened for the erector spinae, the thoracolumbar fascia and part of the gluteus maximus, and the sacral nerves leave through four pairs of openings on each face.