Ulna
Long Bone; appendicular skeleton (limb)
Also known as: Inner forearm bone, Elbow bone
Classification
Long Bone; appendicular skeleton (limb)
Dimensions
A long, slender bone that is usually slightly longer than its neighbor, the radius. It is thickest at the top near the elbow and tapers considerably toward the wrist.
Key Features
The most recognizable feature is the large, hook-shaped notch at the top end that fits into the upper arm bone to create the hinge of the elbow. At the bottom end near the wrist, it terminates in a small, rounded head with a tiny, pointed projection that can be felt as the 'bump' on the outside of the wrist.
Similar Bones
The ulna is most often confused with the radius. To tell them apart, look for the 'wrench-shaped' notch at the top; only the ulna has this distinctive hook for the elbow, whereas the radius has a flat, circular head resembling a button or a nail head.
Across Species
The ulna shows incredible diversity across species. In many fast-running mammals like horses and deer, the ulna is fused with the radius to provide a rigid, stable leg for galloping. In contrast, bats have a highly reduced, thin ulna to keep their wings lightweight. Primates and some tree-climbing mammals retain a separate, well-developed ulna that allows the forearm to rotate, which is essential for grasping branches and manipulating food.
Evolutionary History
The ulna is a foundational part of the tetrapod limb, appearing in the earliest land-dwelling vertebrates. Over millions of years, as different lineages specialized for flying, swimming, or running, the ulna adapted by either lengthening, thickening, or even partially disappearing to suit the mechanical needs of the animal's environment.
Photography Tips
To identify an ulna from a photo, try to capture the large hook-shaped notch at the top. Side-view photos are excellent for showing the gentle 'S' curve of the bone's shaft. Including a common object like a coin or a ruler helps show the overall size and scale of the specimen.