The Sacrum...The Holy Bone
Humans have two hundred or so bones, but we have but one bone that is holy: the sacrum.
The word "sacrum", meaning "sacred" in Latin, lives on in English anatomy as the name for the large heavy bone at the base of the spine.
The Romans called the bone the "os sacrum," which literally meant the "holy bone" and the Greeks termed it the "hieron osteon," the same thing, the "holy bone".
According to the Oxford English Dictionary, the anatomic "sacrum" entered English minus the "os" in 1753. The OED defines "sacrum" with precision (if not concision) as:
The regions of human spine, lest we forget, are the cervical (neck) with 7 vertebrae, the thoracic (upper-back) with 12 vertebrae, the lumbar (lower-back) with 5 vertebrae, and the sacral region with 5 vertebrae.
The last sacral vertebra sits just above the coccyx, which during evolution was the beginning of the tail (and now, it seems, is mainly a bone to bruise or break).
The female sacrum is wider and straighter (less curved) than that of males. This difference in sacral anatomy has evolved because of its value to childbearing.