Sacrum 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. 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
Sacrum 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