Pelvic surface of sacrum

The pelvic surface of the sacrum is concave from the top, and curved slightly from side to side.

Its middle part is crossed by four transverse ridges, which correspond to the original planes of separation between the five sacral vertebrae.

The body of the first segment is large and has the form of a lumbar vertebra; the bodies of the next bones get progressively smaller, are flattened from the back, and curved to shape themselves to the sacrum, being concave in front and convex behind.

At each end of the transverse ridges, are the four anterior sacral foramina, diminishing in size in line with the smaller vertebral bodies.

The foramina give exit to the anterior divisions of the sacral nerves and entrance to the lateral sacral arteries.

Each part at the sides of the foramina is traversed by four broad, shallow grooves, which lodge the anterior divisions of the sacral nerves.

They are separated by prominent ridges of bone which give origin to the piriformis muscle.

If a sagittal section be made through the center of the sacrum, the bodies are seen to be united at their circumferences by bone, wide intervals being left centrally, which, in the fresh state, are filled by the intervertebral discs.