Final answer:
The sacrum is formed by the fusion of five sacral vertebrae and is a crucial part of the pelvis, involved in weight bearing and providing an attachment point for the hip bones.
Step-by-step explanation:
The sacrum is the bone formed by five fused vertebrae and is a key component of the pelvis. It is a triangular-shaped bone that is involved in weight bearing at its broad superior base and then tapers down to a non-weight bearing apex. This fusion of the five sacral vertebrae generally does not begin until after the age of 20. The pelvic girdle includes the sacrum, coccyx, and a single hip bone (coxal bone) on each side, where the hip bones are joined anteriorly and to the axial skeleton at the sacrum. The sacrum's anterior surface has lines of vertebral fusion visible as four transverse ridges, and on the posterior surface, the median sacral crest is a remnant of the fused spinous processes.