The larger of two posterior openings into the bony pelvis. Along the back edge of the ilium and ischium bones, there are two semicircular indentations, the greater and lesser sciatic notches. Two crossed ligaments (the sacrotuberous and sacrospinous ligaments) make these notches into enclosed ovals, the greater and lesser sciatic foramena. The greater sciatic foramen is the upper and larger of these. The gluteal and some hip-joint structures, such as the piriformis muscle and the gluteal nerves and blood vessels, leave the pelvic cavity through this opening, as do the sciatic nerve and the posterior cutaneous nerve of the thigh.