At the ends of the transverse ridges of the pelvic surface 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...

 are seen the anterior sacral foramina (or pelvic 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 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....

 and entrance to the lateral sacral arteries.