骨格の考察7

脊椎のうち５個の仙椎が癒合して生じた骨。骨盤の後壁を作る。その形は長めの菱(ひし)形であるが，十字架になぞらえてラテン語でos sacrum（神聖な骨）と名づけられた。

wiki

In humans, the sacrum (plural: sacrums or sacra) is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx (tailbone). It consists of usually five initially unfused vertebrae which begin to fuse between ages 16–18 and are usually completely fused into a single bone by age 34.

It is curved upon itself and placed obliquely (that is, tilted forward). It is kyphotic—that is,concave facing forward. The base projects forward as the sacral promontory internally, and articulates with the last lumbar vertebra to form the prominent sacrovertebral angle. The central part is curved outward toward the posterior, allowing greater room for thepelvic cavity. The two lateral projections of the sacrum are called ala (wings), and articulate with the ilium at the L-shaped sacroiliac joints.

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