Radius
Long bone of the appendicular skeleton
Also known as: Forearm Bone
Classification
Long bone of the appendicular skeleton
Dimensions
Varies greatly by species, ranging from a few inches in small mammals to several feet in large ungulates. It is generally a slender, slightly curved shaft with a wider upper end and a broader lower end.
Key Features
Look for a long, slightly bowed shaft featuring a distinct round disc-like head at the upper end and a broader, notched lower end designed to connect with wrist bones.
Similar Bones
Often confused with the ulna, which sits right beside it in the limb. You can tell the radius apart by its smaller size in many animals and its distinct, circular top end, whereas the ulna typically has a prominent hook-like grip at its top end.
Across Species
In humans and many other mammals, the radius works alongside the ulna to allow twisting movements of the lower arm. In horses and other running mammals, the radius is fused or heavily modified for stability, losing its twisting ability in favor of straight-line speed. In birds, it is a thin, delicate strut running parallel to the larger ulna.
Evolutionary History
The radius evolved from the fin bones of ancient lobe-finned fish during the transition of vertebrates onto land over 350 million years ago. Across deep time, it has transformed from a weight-bearing paddle support into a specialized lever for twisting and grasping in tetrapods.
Photography Tips
Place the bone against a neutral background with a ruler or coin for scale. Capture a straight-on shot of the entire length, followed by close-ups of both the upper and lower ends to show the joint surfaces clearly. Use soft, directional lighting to highlight the natural curve of the shaft.