Sacrum Stock Photography

In humans, the sacrum is a large, triangular bone at the base of the spine and at the upper, 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 its lower part with the coccyx (tailbone). Usually, it begins as five unfused vertebrae which begin to fuse between the ages of 16–18 years and have usually completely fused into a single bone by the age of 34 years.