Piriformis muscle

Origin: Sacrum

Insertion: Greater trochanter

Artery: Inferior gluteal artery, Lateral sacral artery, Superior gluteal artery

Nerve: Nerve to the Piriformis (S1 and S2 nerve roots)

Action: Rotate laterally (outward) the thigh

Description:
The Piriformis is a flat muscle, pyramidal in shape, lying almost parallel with the posterior margin of the Glutaeus medius. It is situated partly within the pelvis against its posterior wall, and partly at the back of the hip-joint. It arises from the front of the sacrum by three fleshy digitations, attached to the portions of bone between the first, second, third, and fourth anterior sacral foramina, and to the grooves leading from the foramina: a few fibers also arise from the margin of the greater sciatic foramen, and from the anterior surface of the sacrotuberous ligament. The muscle passes out of the pelvis through the greater sciatic foramen, the upper part of which it fills, and is inserted by a rounded tendon into the upper border of the greater trochanter behind, but often partly blended with, the common tendon of the Obturator internus and Gemelli.
Variations.—It is frequently pierced by the common peroneal nerve and thus divided more or less into two parts. It may be united with the Glutaeus medius, or send fibers to the Glutaeus minimus or receive fibers from the Gemellus superior. It may have only one or two sacral attachments or be inserted in to the capsule of the hip-joint. It may be absent.