The Ischium

The ischium (Os ischii) forms the posterior part of the ventral wall or floor of the bony pelvis. It slopes a little downward and inward, but is practically horizontal in the longitudinal direction. It is irregularly quadrilateral, and may be described as having two surfaces, four borders, and four angles.

The superior or pelvic surface (Facies pelvina) is smooth and slightly concave from side to side.

The inferior surface (Facies externa) is nearly flat, and is in great part roughened for the attachment of the adductor muscles.

The anterior border forms the posterior margin of the obturator foramen.

The posterior border is thick and rough. It slopes obliquely inward and forward to meet the border of the other side, forming with it the ischial arch (Arcus ischiadicus).

The internal border meets the opposite bone at the symphysis.

The external border is thick and rounded, but concave in its length; it forms the lesser sciatic notch, the lower boundary of the lesser sciatic foramen.

The antero-internal angle or symphyseal branch (Ramus symphyseos) meets the pubis, with which it forms the inner boundary of the obturator foramen.

The antero-external angle or acetabular branch joins the other two bones at the acetabulum, of which it forms more than half. Superiorly it bears part of the superior ischiatic spine (Spina ischiadica), and internally it is grooved for the obturator vessels.

The postero-external angle is a thick three-sided mass, the tuber ischii (Tuber ischiadicum); its lower border is the inferior ischiatic spine, to which the biceps femoris and semitendinosus muscles are attached.