The structure indicated is the greater sciatic notch of the pelvis.

The greater sciatic notch les between the ischial spine (inferiorly) and the posterior inferior iliac spine (superiorly). The sacrospinous ligament which attaches from the ischial spine to the lateral aspect of the sacrum and coccyx converts the greater sciatic notch into the greater sciatic foramen.

The lesser sciatic notch lies below the ischial spine, and is converted into the lesser sciatic foramen by the sacrospinous and sacrotuberous ligaments.

The piriformis muscle passes through the greater sciatic foramen and divides it into two spaces:

• Greater sciatic foramen above the piriformis (suprapiriform foramen)
• Greater sciatic foramen below the piriformis (infrapiriform foramen)

The following structures pass in the foramen above the piriformis:

• Superior gluteal vessels
• Superior gluteal nerve

The following structures pass in the foramen below the piriformis:

• Inferior gluteal vessels
• Internal pudendal vessels
• Inferior gluteal nerve
• Pudendal nerve
• Sciatic nerve
• Posterior femoral cutaneous nerve
• Nerve to obturator internus
• Nerve to quadratus femoris

Learn about the bones of the pelvis in this anatomy tutorial.