The obturator internus muscle, like the piriformis muscle, is a muscle of the pelvic wall and of the gluteal region (Fig. 6.45). It is a flat fan-shaped muscle originating from the medial surface of the obturator membrane and adjacent bone of the obturator foramen (see Chapter 5, pp. 442–443). Because the pelvic floor attaches to a thickened band of fascia across the medial surface of the obturator internus, the obturator internus forms: the anterolateral wall of the pelvic cavity above the pelvic floor, and the lateral wall of the ischio-anal fossa in the perineum below the pelvic floor.