The wing of ilium (or ala) is the large expanded portion which bounds the greater pelvis laterally. It presents for examination two surfaces—an external and an internal—a crest, and two borders—an anterior and a posterior.

The external surface, known as the dorsum ilii, is directed backward and lateralward behind, and downward and lateralward in front.

It is smooth, convex in front, deeply concave behind; bounded above by the crest, below by the upper border of the acetabulum, in front and behind by the anterior and posterior borders.

This surface is crossed in an arched direction by three lines—the posterior, anterior, and inferior gluteal lines.

The posterior gluteal line (superior curved line), the shortest of the three, begins at the crest, about 5 cm. in front of its posterior extremity; it is at first distinctly marked, but as it passes downward to the upper part of the greater sciatic notch, where it ends, it becomes less distinct, and is often altogether lost.

Behind this line is a narrow semilunar surface, the upper part of which is rough and gives origin to a portion of the Glutæus maximus; the lower part is smooth and has no muscular fibers attached to it.