Dorsal surface of sacrum

The dorsal surface of the sacrum is convex and narrower than the pelvic surface.

In the middle line is the median sacral crest, surmounted by three or four tubercles—the rudimentary spinous processes of the upper three or four sacral vertebrae.

On either side of the median sacral crest is a shallow sacral groove, which gives origin to the multifidus muscle.

The floor of the groove is formed by the united laminae of the corresponding vertebrae.

The laminae of the fifth sacral vertebra, and sometimes those of the fourth, do not meet at the back, resulting in a fissure known as the sacral hiatus in the posterior wall of the sacral canal.

The sacral canal is a continuation of the spinal canal and runs throughout the greater part of the sacrum.

Above the sacral hiatus, it is triangular in form.

The canal lodges the sacral nerves, via the anterior and posterior sacral foramina.