Sacrospinous ligament

The sacrospinous ligament is thin, and triangular in form; it is attached by its apex to the spine of the ischium, and medially, by its broad base, to the lateral margins of the sacrum and coccyx, in front of the sacrotuberous ligament with which its fibers are intermingled.

Relations.—It is in relation, anteriorly, with the Coccygeus muscle, to which it is closely connected; posteriorly, it is covered by the sacrotuberous ligament, and crossed by the internal pudendal vessels and nerve. Its upper border forms the lower boundary of the greater sciatic foramen; its lower border, part of the margin of the lesser sciatic foramen.

These sacrotuberous and sacrospinous ligaments convert the sciatic notches into foramina.

• The greater sciatic foramen is bounded, in front and above, by the posterior border of the hip bone; behind,by the sacrotuberous ligament; and below, by the sacrospinous ligament. It is partially filled up, in the recent state, by the Piriformis which leaves the pelvis through it. Above this muscle, the superior gluteal vessels and nerve emerge from the pelvis; and below it, the inferior gluteal vessels and nerve, the internal pudendal vessels and nerve, the sciatic and posterior femoral cutaneous nerves, and the nerves to the Obturator internus and Quadratus femoris make their exit from the pelvis.
• The lesser sciatic foramen is bounded, in front, by the tuberosity of the ischium; above, by the spine of the ischium and sacrospinous ligament;behind, by the sacrotuberous ligament. It transmits the tendon of the Obturator internus, its nerve, and the internal pudendal vessels and nerve.