Clues to look for:
1. it is big, triangular. It is made of 5 fused vertebrae.
2. it contains 8 see-through holes passages for nerves and blood vessels. The anterior openings are called anterior (= ventral; = pelvic) sacral foramina and the posterior openings are called posterior (= dorsal) sacral foramina.

The sacral canal is the sacral part of the vertebral canal.

There is a gap in the middle of the posterior part of the arch of the 5th sacral vertebrae. This means that there is an entrance to the vertebral canal. This entrance is called the sacral hiatus.

Epidural anesthesia are injected in the sacral hiatus or in the posterior sacral foramina.