The fascia of the Piriformis is very thin and is attached to the front of the sacrum

Sacrum

In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...

 and the sides of the greater sciatic foramen

Greater sciatic foramen

-Boundaries:It is bounded as follows:* anterolaterally by the greater sciatic notch of the illium* posteromedially by the sacrotuberous ligament* inferiorly by the sacrospinous ligament and the ischial spine* superiorly by the anterior sacroilliac ligament...

; it is prolonged on the muscle into the gluteal region.

At its sacral attachment around the margins of the anterior sacral foramina

Anterior sacral foramina

At the ends of the transverse ridges of the pelvic surface of the sacrum are seen the anterior sacral foramina , four in number on either side, somewhat rounded in form, diminishing in size from above downward, and directed lateralward and forward.They give exit to the anterior divisions of the...

 it comes into intimate association with and ensheathes the nerves emerging from these foramina.

Hence the sacral nerves

Sacral nerves

The five sacral nerves emerge from the sacrum. Although the vertebral components of the sacrum are fused into a single bone, the sacral vertebrae are still used to number the sacral nerves....

 are frequently described as lying behind the fascia.

The internal iliac vessels and their branches, on the other hand, lie in the subperitoneal tissue in front of the fascia, and the branches to the gluteal region emerge in special sheaths of this tissue, above and below the Piriformis muscle

Piriformis muscle

The piriformis is a muscle in the gluteal region of the lower limb. It was first named by Spigelius, a professor from the University of Padua in the 16th century.- Origin and insertion :...

.