Final answer:
The sacrum consists of five fused sacral vertebrae, forming a single bone in the adult vertebral column. This fusion is marked by transverse ridges and results in the formation of the median and lateral sacral crests.
Step-by-step explanation:
The sacrum is a triangular-shaped bone located near the inferior end of the adult vertebral column. It consists of five fused sacral vertebrae, which form a single bone in the adult vertebral column. These five sacral segments are fused in a process that begins around the age of 20 and is not completed until middle age. This fusion creates several notable features on the sacrum, including the median sacral crest formed from the fused spinous processes and the lateral sacral crest formed from the fused transverse processes. The lines of vertebral fusion can often be seen on the anterior surface of the sacrum as transverse ridges.