Ulna
Long bone, part of the appendicular skeleton
Also known as: Forearm bone, inner forearm bone, elbow bone
Classification
Long bone, part of the appendicular skeleton
Dimensions
Ranges greatly in length depending on the animal species, typically featuring a noticeably enlarged, wrench-like upper end that tapers down into a slender shaft toward the lower end.
Key Features
The standout feature is the large, C-shaped or wrench-shaped notch at the top end, designed to act as a hinge for the elbow joint. It also features a prominent backward-projecting hook at the top that forms the point of the elbow.
Similar Bones
Often confused with the radius, which sits right beside it in the limb. You can tell the ulna apart by its signature wrench-shaped top end, whereas the radius typically has a round, button-like top surface designed for spinning.
Across Species
In humans and many climbing or grasping mammals, the ulna works alongside the radius to allow the forearm to twist. In running animals like horses and deer, the ulna is heavily fused with the neighboring radius to maximize stability and prevent twisting during high-speed sprints. In birds, the ulna is a robust bone in the wing that provides the main anchorage point for secondary flight feathers.
Evolutionary History
The two-bone forearm arrangement of an upper bone and a lower pair of parallel bones dates back hundreds of millions of years to the earliest tetrapods crawling onto land from ancient waters, evolving from the sturdy fin bones of lobe-finned fish.
Photography Tips
Lay the bone flat against a solid, neutral-colored background. Take the primary shot from the side to clearly capture the C-shaped wrench feature at the top. Include a small coin or ruler next to the bone to show proper scale.