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Related to os sacrum: sacral, sacral bone
sa·cra(sā'krŭm, sā'kră), [TA]
The segment of the vertebral column forming part of the pelvis; a broad, slightly curved, spade-shaped bone, thick above, thinner below, closing in the pelvic girdle posteriorly; it is formed by the fusion of five originally separate sacral vertebrae; it articulates with the last lumbar vertebra, the coccyx, and the hip bone on either side.
[L. (lit. sacred bone), neuter of sacer (sacr-), sacred]