The Ilium

The ilium (Os ilium) is the largest of the three parts. It is irregularly triangular and presents two surfaces, three borders, and three angles.

The gluteal surface (Facies glutæa) faces upward, backward, and outward. It is wide and concave in front, narrower and convex behind. The wide part is crossed by the curved gluteal line (Linea glutæa), which extends from the middle of the inner border toward the external angle. This surface gives attachment to the middle and deep gluteal muscles.

The pelvic surface (Facies pelvina) faces in the opposite direction; it is convex, and consists of two distinct parts. The inner triangular part (Pars articularis) is roughened for ligamentous attachment, and bears an irregular facet, the auricular surface (Facies auricularis), for articulation with the sacrum. The outer quadrilateral part (Pars iliaca) is in general smooth. It is crossed by the ilio-pectineal line (Crista iliopectinea), which begins below the auricular surface and is continued on the shaft of the bone to join the anterior border of the pubis. The line is interrupted by furrows for the iliaco-femoral vessels, and below these it bears the psoas tubercle (Tuberculum psoadicum), which gives attachment to the psoas minor muscle. The iliacus muscle is attached to the surface external to the ilio-pectineal line.

The anterior border or crest (Crista iliaca) is concave, thick, and rough.

The internal border is deeply concave; its middle part forms the lower boundary of the great sciatic foramen, and it is continuous behind with the superior ischiatic spine.

The external border is concave and in great part rough. Its fore part is crossed by grooves for the ilio-lumbar vessels, which are continued on the pelvic surface. The nutrient foramen is usually situated on or near the posterior part of this border.

The internal or sacral angle (Tuber sacrale) curves upward and a little backward close to the first sacral spine, and forms here the highest point of the skeleton. It is somewhat thickened and rough.

The external or coxal angle (Tuber coxæ) forms the basis of the point of the hip. It is a large quadrangular mass, narrow in its middle, and enlarged at either end, where it bears a pair of tuberosities. It is roughened for muscular attachment.

The posterior or acetabular angle meets the other two bones at the acetabulum, of which it forms about two-fifths. Its prominent upper border forms part of the superior ischiatic spine, which is roughened externally, smooth internally. Two depressions above and in front of the acetabulum give attachment to the tendons of origin of the rectus femoris muscle. This angle is connected with the ala or wide part of the bone by a constricted part, often termed the shaft. The latter is of three-sided prismatic form. Its external surface is convex and rough, and gives attachment to the deep gluteus muscle. Its pelvic surface is smooth and is grooved for the obturator vessels and nerves. Its ventral surface is crossed by vascular grooves, below which is a rough area, bounded internally by the psoas tubercle.