Ischium

The body of the ischium forms the posteroinferior part of the acetabulum. On its posterior border projects the triangular ischial spine, which separates the greater sciatic notch above from the lesser sciatic notch below and anchors the sacrospinous ligament and coccygeus. Below the lesser notch lies the large, rough ischial tuberosity — the "sitting bone" — which gives origin to the hamstrings (semimembranosus, semitendinosus, long head of biceps femoris) and the ischial part of adductor magnus, and attachment to the sacrotuberous ligament. The ischial ramus passes forward to fuse with the inferior pubic ramus, forming the ischiopubic ramus that bounds the obturator foramen and the pubic arch and gives attachment to the crura of the penis/clitoris and the perineal membrane.