Final answer:
The sacrum, located at the inferior end of the vertebral column, is formed by the fusion of five sacral vertebrae, which create structures like the median sacral crest from fused spinous processes and the lateral sacral crest from fused transverse processes.
Step-by-step explanation:
The Sacrum and Its Components
The sacrum is a single bone located near the inferior end of the adult vertebral column.
It is formed by the fusion of five sacral vertebrae, which contribute to the posterior portion of the pelvis.
The fusion of these vertebrae does not begin until after the age of 20.
Once these bones are fused, they form several notable structures including the median sacral crest, composed of the fused spinous processes, and the lateral sacral crest, which originates from the fused transverse processes of the sacral vertebrae.
The anterior surface of the older adult sacrum reveals the lines of vertebral fusion as four transverse ridges, and several ligaments support and immobilize the sacrum, especially at the sacroiliac joints.