Sacrum
Irregular bone, part of the core axial skeleton
Also known as: Tailbone foundation, rump bone
Classification
Irregular bone, part of the core axial skeleton
Dimensions
Wedge-shaped block typically ranging from four to six inches in length, featuring a broad top tapering down to a narrow bottom tip.
Key Features
Characterized by its distinct wedge shape, a smooth curved front face, a bumpy and ridged back face, and neat rows of prominent holes on both sides where nerve pathways once passed.
Similar Bones
Can be mistaken for a pelvic hip bone at a glance, but the sacrum is symmetrical with a distinct wedge outline and characteristic pairs of holes, whereas hip bones are sprawling and asymmetrical.
Across Species
In four-legged animals, the sacrum anchors the tail and ties the hind legs directly to the spine, bearing immense propulsion forces. In humans, who stand upright, it acts as a keystone in a natural arch, supporting the weight of the upper body and transferring it smoothly to the hips.
Evolutionary History
The sacrum evolved as independent vertebrae fused together to form a sturdy platform, a necessary transition as early land-dwellers shifted from side-to-side swimming motions to weight-bearing limbs that could push off the ground.
Photography Tips
Take photos from the top-down view to show the entire wedge shape clearly, and include a small household object next to it for scale. Use side lighting to bring out the texture of the bumpy ridges and the depth of the holes.