The iliac fossa is a large, smooth, concave surface on the internal surface of the ilium (part of the 3 fused bones making the hip bone). The fossa is bounded above by the iliac crest, and below by the arcuate line; in front and behind, by the anterior and posterior borders of the ilium.The iliacus and nearby musclesIliac fossaIliac fossaHip joint. Lateral view. Iliac fossaLumbar and sacral plexus. Deep dissection.Anterior view The iliac fossa is a large, smooth, concave surface on the internal surface of the ilium (part of the 3 fused bones making the hip bone). The fossa is bounded above by the iliac crest, and below by the arcuate line; in front and behind, by the anterior and posterior borders of the ilium. The fossa gives origin to the Iliacus muscle and is perforated at its inner part by a nutrient canal; below this there is a smooth, rounded border, the arcuate line, which runs anterior, inferior, and medial. When the 'left' or 'right' adjective is used (e.g 'right iliac fossa'), the iliac fossa usually means one of the inguinal regions of the nine regions of the abdomen.