Radius
Long bone, part of the limb (appendicular) skeleton.
Also known as: Lower Arm Bone
Classification
Long bone, part of the limb (appendicular) skeleton.
Dimensions
In many animals, it is roughly as long as the upper arm bone but more slender. In humans, it typically measures between 8 and 11 inches in length, featuring a flared bottom and a disk-like top.
Key Features
Look for a very distinctive, flat, circular head at the top that looks like a small button or a nail head. The shaft has a sharp ridge running along the inner side and flares out into a wide, chunky base at the bottom with a small pointed tip on the very end.
Similar Bones
It is most often confused with the ulna, which is its partner in the lower arm. You can tell them apart because the ulna has a large C-shaped notch at the top (like a wrench) while the radius has a flat, circular top. It might also be confused with the fibula from the lower leg, but the radius is much thicker and has more complex ends.
Across Species
The radius is the star of specialized movement. In humans and primates, it can actually rotate around its partner (the ulna) to flip the palm upward or downward. In animals like horses or deer, where stability is more important than rotation, this bone is often fused with the ulna to create a rigid, shock-absorbing pillar for running. In birds, it acts as a lightweight structural support for the wing.
Evolutionary History
The radius first appeared in prehistoric lobe-finned fish as a support for the fins. Over millions of years, as vertebrates moved onto land, it evolved from a simple strut into a dynamic lever that allows tetrapods to support their weight and navigate complex environments. Its ability to rotate independent of the ulna was a major evolutionary leap for climbing and tool-using mammals.
Photography Tips
Lay the bone on a flat surface and take one photo from directly above. Then, take close-up shots of the circular top and the flared bottom. Including a ruler or a common object like a coin can help viewers understand the size and scale of the specimen.