Radius
Long bone of the appendicular skeleton
Also known as: Outer forearm bone, turning bone
Classification
Long bone of the appendicular skeleton
Dimensions
Varies widely by species, but generally slender with a narrow, slightly curved shaft. It is shorter and more robust than the companion bone in the lower front limb.
Key Features
Look for a long, slender shaft with a noticeable gentle curve along its length. One end features a small, round, cup-like depression that fits neatly into the elbow joint, while the wider lower end broadens out to connect with the wrist bones, often showing small grooves where tendons pass over it.
Similar Bones
Beginners frequently confuse the radius with the ulna, its companion bone in the lower limb. While the ulna usually features a prominent hooked grip at the top for the elbow, the radius is distinguished by its smoother, round head at the top and its slightly flared lower end designed for the wrist.
Across Species
In humans and many primates, the radius is specially adapted to cross over the ulna, allowing the hand and palm to flip upward and downward. In running animals like horses and deer, the radius is fused or rigidly locked parallel to its companion bone to provide maximum stability during high-speed galloping, sacrificing twisting ability for straight-line strength. In birds, it acts as a slender strut working alongside the ulna to support the wing feathers.
Evolutionary History
The radius traces its origins back to the paired fin bones of ancient lobe-finned fish that first ventured onto land during the Devonian period. Over hundreds of millions of years, the basic blueprint of one upper arm bone followed by two lower arm bones has been wonderfully modified through evolution to support swimming in whales, flying in bats, running in cheetahs, and grasping in primates.
Photography Tips
To get a clear identification photo, lay the bone flat against a neutral, high-contrast background with a ruler or coin placed nearby for scale. Take one overall shot showing the entire length from top to bottom, followed by close-up shots focusing on the distinctive shapes of both the upper and lower ends.