Ulna
Long bone, part of the appendicular skeleton (limbs).
Also known as: Forearm bone, Elbow bone
Classification
Long bone, part of the appendicular skeleton (limbs).
Dimensions
In mammals, it is typically long and slender, often slightly longer than its companion bone in the lower arm. Its length varies significantly depending on the species' size, from a few centimeters in small rodents to over half a meter in large hoofed animals.
Key Features
The most defining feature is the 'wrench' or 'hook' shape at the top end, which forms the point of the elbow. This notch fits perfectly around a spool-shaped joint on the upper arm bone. There is also usually a prominent ridge running down the shaft for muscle attachment.
Similar Bones
It is most frequently confused with the Radius, which is its partner bone in the forearm. You can tell them apart because the ulna has a distinct hook-like shape at the top, while the radius typically has a flat, circular, button-like head.
Across Species
The ulna shows incredible variety across nature. In digging animals like moles, it is short and broad to support powerful muscles. In running animals like horses and deer, the lower part of the ulna often fuses with the radius to save weight and provide stability. In bats, the ulna is extremely thin and reduced, as the radius carries most of the flight stress.
Evolutionary History
This bone can be traced back to the lobe-finned fish that first crawled onto land. Over millions of years, it transitioned from a fin support to a load-bearing lever. In early tetrapods, it was thick and heavy, but as mammals evolved more specialized movements like climbing or running, it became more slender and refined to allow for the rotation of the hand.
Photography Tips
To get the best identification, place the bone on a plain, neutral background like a gray towel or piece of wood. Take one photo from the front and another from the side to show the curve. Always include a common object like a coin or a ruler next to the bone to provide a clear sense of its size.