Ulna
Long Bone, Appendicular Skeleton
Also known as: Forearm Bone, Elbow Bone
Classification
Long Bone, Appendicular Skeleton
Dimensions
In small mammals such as rodents or small carnivores, this bone typically measures between 2 and 8 centimeters in length. It is slender and elongated, featuring a distinctive hook-like projection at the upper end that forms the point of the elbow.
Key Features
The single most diagnostic feature of this bone is the prominent, wrench-shaped or C-shaped notch at the upper end, which cradles the lower end of the upper arm bone. Just above this notch sits a protruding point that forms the elbow joint. The shaft generally tapers into a slender, slightly flattened rod toward the lower end.
Similar Bones
It is most frequently confused with the adjacent forearm bone, the radius. However, the radius lacks the deep hook-like notch and instead features a rounded, button-like head at its upper end. It can also be confused with the lower leg bone (fibula), but the fibula is much more uniformly thin and lacks any large, wrench-shaped notch at its top.
Across Species
The structure of this bone varies dramatically depending on how an animal moves. In bats, it is extremely reduced to a paper-thin sliver, leaving the neighboring forearm bone to support the wing membrane. In birds, it is robust and often lined with tiny, evenly spaced bumps called quill knobs where flight feathers attach. In horses and deer, it fuses completely with the adjacent forearm bone to create a solid, shock-absorbing column for running. In humans, cats, and bears, it remains entirely separate, allowing the forearm to rotate freely.
Evolutionary History
This bone has a long evolutionary lineage originating in early lobe-finned fish, where simple bony elements supported the pectoral fins. As ancient tetrapods transitioned onto land over 370 million years ago, these elements elongated into functional lever arms to bear weight. Over deep time, the upper end developed a prominent bony hook, providing leverage for powerful muscles that allow animals to push off the ground, dig, climb, or swim.
Photography Tips
To capture the most diagnostic details, photograph the specimen from the side to clearly highlight the deep, C-shaped notch near the top. Place a standard ruler or a common coin beside the bone for scale, and use low, angled lighting to cast soft shadows that emphasize the subtle ridges and contours along the shaft.