Question

The combining form sacr/o refers to the sacrum, the triangular bone below the lumbar vertebrae. Five sacr+al vertebrae are present at birth, but in the adult, they are fused to form one bone. The sacrum is the result of fusion of five _____ bones.

Answer 1

The sacrum is the result of the fusion of five sacral vertebrae, creating a single bone that forms the posterior portion of the pelvis and supports the adult vertebral column. This fusion begins after age 20 and results in transverse ridges on the anterior surface and the median and lateral sacral crests on the posterior surface.

Step-by-step explanation:

The sacrum is a single bone located near the inferior end of the adult vertebral column that is formed by the fusion of five sacral vertebrae. At birth, these vertebrae are distinct, but as a person matures, they gradually fuse together to form one triangular-shaped bone. This process, which contributes to forming the posterior portion of the pelvis, typically does not begin until after the age of 20. Once fused, the sacrum supports the weight of the body and connects the spine to the hip bones.

On an older adult's sacrum, anteriorly, one can observe four transverse ridges that indicate the lines of vertebral fusion. Posteriorly, the median sacral crest formed from the fused spinous processes runs down the midline, and the lateral sacral crest arises from the fused transverse processes. These features provide attachment points for muscles and ligaments.

The sacrum plays a critical role in the vertebral column, as it helps to support the head, allows attachment for ribs and muscles of the back and neck, protects the spinal cord, and forms a key part of the structure of the pelvis.