Final answer:
The structure immediately deep to the sacrospinous ligament is the internal pudendal vessels and pudendal nerve. They pass through the lesser sciatic foramen, which is outlined in part by the sacrospinous ligament.
Step-by-step explanation:
The structure immediately deep to the sacrospinous ligament is internal pudendal vessels and pudendal nerve. The sacrospinous ligament, along with the sacrotuberous ligament, contributes to the formation of the greater and lesser sciatic foramina. Within the lesser sciatic foramen, the internal pudendal vessels and pudendal nerve travel to reach the perineum. The sacrospinous ligament spans from the sacrum to the ischial spine, helping to define the boundaries of these foramina through which neurovascular structures pass.