saculum

The sacrum, also known as the sacral bone or sacrum, is a triangular bone located in the pelvis of vertebrates. It is formed by the fusion of five vertebrae, known as sacral vertebrae, and is situated between the ilium and the coccyx. The sacrum plays a crucial role in supporting the weight of the upper body and transmitting forces from the legs to the spine.

In humans, the sacrum is typically composed of five fused vertebrae, although variations can occur. It articulates with the ilium of the pelvis and the coccyx at the sacrococcygeal joint. The sacrum also houses the sacral canal, which contains the sacral nerves and the cauda equina, a collection of nerve roots.

The sacrum is a key landmark in anatomy and is often used as a reference point for various medical procedures and surgeries. Its robust structure and strategic location make it an essential component of the skeletal system.