The structure indicated is the piriformis muscle.

The piriformis muscle is one of the muscles of the gluteal region. The muscles in the gluteal region are divided into a superficial and deep group. There are four muscles of the superficial group:

1. Gluteus maximus
2. Gluteus medius
3. Gluteus minimus
4. Tensor fasciae latae

There are 5 muscles in the deep group:

1. Gemellus superior
2. Gemellus inferior
3. Quadratus femoris
4. Piriformis
5. Obturator internus

The deep muscles of the gluteal region generally serve to laterally rotate the femur.

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)

Origin: Anterior portion of lateral process of sacrum. Gluteal surface of ilium at border of greater sciatic notch

Insertion: Superior border of greater trochanter

Action: Lateral rotation of hip. Assists abduction of hip when hip is flexed.

Innervation: Nerve to piriformis

Learn more about the anatomy of the gluteal muscles in this tutorial.