Sacrum
The sacrum 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).
The sacrum is located just above the buttocks, at the upper end of the gluteal cleft. The somewhat flattened skin area over the sacrum is known as the sacral triangle.
The sacrum consists of 5 verterbrae that start out as separate bones at birth and fuse by adulthood into one bone. It is at the lower portion of the vertebral column, below the lumbar vertebrae.
Sexual dimorphism
The sacrum is noticeably sexually dimorphic (differently-shaped in males and females). In the female the sacrum is shorter and wider than in the male; the lower half forms a greater angle with the upper; the upper half is nearly straight, the lower half presenting the greatest amount of curvature. The bone is also directed more obliquely backward; this increases the size of the pelvic cavity and renders the sacrovertebral angle more prominent. In the male, the curvature is more evenly distributed over the whole length of the bone, and is altogether greater than in the female.
See also
These photos are presented for the purposes of identifying various body parts
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