The structure indicated is the sacrotuberous ligament.
The sacrotuberous ligament runs from the sacrum (sacro-) to the ischial tuberosity (-tuberous). The sacrotuberous ligament contains the coccygeal branch of the inferior gluteal artery. In combination with the sacrospinous ligament, the two ligaments serve to form two important foramina which allow structures to exit the pelvis:
- Greater sciatic foramen
- Lesser sciatic foramen
The sacrospinous ligament runs from the sacrum and coccyx to the ischial spine. The greater sciatic foramen is formed by the sacrospinous ligament and the greater sciatic notch; the lesser sciatic foramen is formed by the sacrotuberous ligament and the lesser sciatic notch.
Sometimes the pudendal nerve can become trapped between the sacrotuberous ligament and the sacrospinous ligament, causing perineal pain (pudendal nerve entrapment/pudendal neuralgia).
Learn all about the anatomy of the pelvis in this tutorial.