The sacrum is not part of the appendicular skeleton; it is classified as part of the axial skeleton. The axial skeleton includes the skull, vertebral column, and rib cage, serving to protect the brain, spinal cord, and vital organs in the thorax.
The sacrum itself is a triangular bone formed by the fusion of five vertebrae and is located at the base of the vertebral column. It connects the spine to the hip bones (ilia) through the sacroiliac joints, which are crucial for weight-bearing and locomotion.
On the other hand, the appendicular skeleton consists of the bones of the limbs and the supporting girdles (shoulder girdle and pelvic girdle) that attach them to the axial skeleton. Therefore, while the sacrum plays an essential role in the skeletal system by providing stability and support, it is not categorized under the appendicular skeleton.