Radius
Long Bone, part of the limb (appendicular) skeleton
Also known as: Lower Arm Bone, Radial Bone
Classification
Long Bone, part of the limb (appendicular) skeleton
Dimensions
A slender, slightly curved bone that is shorter than its partner in the lower arm. It typically features a straight shaft with a distinct disk-like top and a wider, flared bottom.
Key Features
Look for a flat, circular 'head' at the top that resembles a nail head or a spool. The shaft is often slightly bowed, and the bottom end flares out into a chunky, wide base with a smooth surface where it meets the wrist.
Similar Bones
Often confused with the Ulna, its companion bone. You can tell them apart because the Ulna has a very large, wrench-shaped hook at the top, whereas the Radius has the flat, round disk. It might also be mistaken for a Fibula from the lower leg, but the Radius is much sturdier and has more complex, flared ends.
Across Species
The radius is the primary weight-bearing bone of the lower front limb in many mammals. In humans and primates, it allows for the unique ability to rotate the forearm, crossing over the ulna to turn the palm up or down. In contrast, in hoofed animals like horses or deer, the radius is often fused with the ulna to provide a rigid, stable column for high-speed running. In birds, the radius acts as a lightweight strut within the wing, supporting the feathers during flight.
Evolutionary History
Tracing back to the first tetrapods that crawled onto land, the radius emerged as one of the two parallel bones of the front limb. Over millions of years, its role has shifted dramatically based on the animal's needs—becoming robust for digging in moles, elongated for steering in whales, or specialized for delicate manipulation in climbing mammals.
Photography Tips
Lay the bone on a flat, neutral surface with a ruler alongside for scale. Take one photo from directly above to show the overall curve, and a close-up 'top-down' photo of the round disk-like head to confirm the identification.