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 moves anteriorly, "counter-nutation". In upright vertebrates, the sacrum is capable of slight independent movement along the sagittal plane. On bending backward the top (base) of the sacrum moves forward relative to the ilium; on bending forward the top moves back. The "sacrum" refers to all of the parts combined. Its parts are called "sacral vertebrae" when referred individually. In some cases the sacrum will consist of six pieces or be reduced in number to four. The bodies of the first and second vertebrae may fail to unite. Sometimes the uppermost transverse tubercles are not joined to the rest of the