Description: The piriformis m. originates with a broad surface from the anterior, middle third of the sacrum, S2-S4. It narrows and courses laterally, traversing the greater sciatic foramen, inserting on the greater trochanter of the femur.  This muscle is a landmark because most structures usually leave the pelvis inferior to the piriformis; only the superior gluteal vessels and nerves exit above it. Circulation is via the Inferior gluteal a., and the innervation is S1-S2 via the  nerve to the piriformis.