The ilium fuses by means of its short, thick, inferior part, called the body (corpus ossis ilii), with the other parts of the hip bone in the region of the acetabulum; the superior fan-shaped and fairly thin part of the ilium forms the wing or ala (ala ossis ilii). The relief of the bone is mainly deter­mined by the muscles under the effect of which crests, lines, and spines formed at the sites of tendon attachment, and fossae formed where the muscles orig­inated. The superior free border of the wing, for instance, is a sinuous crest (crista iliaca) to which three broad abdominal muscles are attached. The crest ends anteriorly as the anterior superior iliac spine (spina iliaca anterior superior) and posteriorly as the anterior posterior iliac spine (spina iliaca posterior superior). Below each of these spines are another two spines on the anterior and posterior iliac borders, the anterior and posterior inferior iliac spines (spina iliaca anterior inferior and spina iliaca posterior inferior). The inferior spinae are separated from the superior spinae by notches. Below and to the front of the anterior inferior spine at the junction of the ilium with the pubis is the iliopubic eminence (eminentia iliopectinea) and below the posterior inferior spine is the deep greater sciatic notch (incisura ischiadica major), continuous downward with the ischial spine (spina ischiadica), which is on the ischium. The inner (medial) surface of the iliac wing is smooth, slightly cancave, and forms the iliac fossa (fossa iliaca) produced from sup­porting the viscera in vertical posture of the body. Posterior to and below the fossa is an ear-shaped articular surface, auricular surface (facies auri-cularis) the site of articulation with the corresponding surface of the sacrum. Behind and above the auricular surface is the iliac tuberosity (tuberositas iliaca) giving attachment to the interosseous sacro-iliac ligaments. The iliac fossa is separated from the medial surface of the distally located iliac body by an arched edge, the arcuate line (linea arcuata). Occasionally conspicuous rough lines, marks of the origin of the gluteal muscles, are seen on the external (lateral) surface of the iliac wing.