Radius
Long Bone; appendicular skeleton (limb)
Also known as: Forearm bone, thumb-side bone
Classification
Long Bone; appendicular skeleton (limb)
Dimensions
A medium-length, relatively slender bone of the lower arm. It typically thickens at the lower end near the wrist and has a distinctive circular top.
Key Features
The most recognizable feature is the 'radial head', which looks like a small, flat-topped wheel or a spool at the top end. The shaft is slightly curved, and the bottom end is wide and block-like where it meets the wrist.
Similar Bones
It is most frequently confused with the ulna, which sits right next to it. You can tell them apart because the ulna has a large, hook-shaped top (like a wrench) while the radius has the flat, circular top.
Across Species
In many mammals, the radius is the primary weight-bearing bone of the front limb. In horses and cattle, it is often fused with the ulna for stability during running. In contrast, in tree-climbing animals and humans, it remains separate and mobile, allowing for the unique ability to rotate the hand—a movement called supination.
Evolutionary History
The radius originated from the paired fins of lobe-finned fish. As vertebrates moved onto land, this bone adapted to provide more structural support while eventually gaining the ability to cross over its partner bone, the ulna. This rotation was a key evolutionary development for grasping and complex tool use.
Photography Tips
To help with identification, take a clear photo of both ends. The circular top view is the most important angle. Place the bone on a neutral background next to a ruler or a common object like a coin to show its size.
Notes
what is the highlighed part