Ulna
Long bone of the appendicular skeleton
Also known as: Elbow bone, Inner forearm bone
Classification
Long bone of the appendicular skeleton
Dimensions
In mammals, the ulna is typically longer than the other forearm bone (the radius) and features a stout, rectangular top end that fits into the upper arm bone. It is generally slender through the shaft, tapering toward the bottom end.
Key Features
The most distinctive part of the ulna is the top end, which features a deep, C-shaped scoop that looks remarkably like a crescent wrench or a bottle opener. This scoop acts as a hinge. On the back of this scoop is a large, blunt projection—the 'point' of your elbow—which serves as a powerful attachment point for the muscles that straighten the arm.
Similar Bones
The ulna is most often confused with the radius (the other forearm bone). You can tell them apart because the radius has a round, flat top that looks like a button or a nail head, whereas the ulna has the deep, hook-like C-shaped scoop. Larger mammal ulnas might be mistaken for fragments of the lower leg bones, but the specific 'wrench' shape at the joint is unique to the elbow.
Across Species
The ulna shows incredible variation across the animal kingdom. In hoofed animals like horses or deer, the ulna is often fused to the radius to provide a rigid, stable leg for running. In contrasts, primates and many climbing mammals have a free-moving ulna that allows the forearm to rotate, which is essential for grasping branches or using tools. In bats, the ulna is highly reduced to a thin splinter to keep the wing light for flight.
Evolutionary History
Tracing back to the first lobe-finned fishes that crawled onto land, the ulna emerged as one of the two supporting pillars of the front limb. Over millions of years, it has been modified from a heavy weight-bearing strut in early tetrapods into a specialized lever that allows for everything from the powerful digging of moles to the delicate manipulative skills of humans.
Photography Tips
To identify an ulna, capture a clear side-view of the C-shaped scoop. It is also helpful to photograph the bone resting flat on a neutral background next to a ruler or common object like a coin to show its scale. Make sure the lighting isn't too harsh so the texture of the joint surface is visible.