Ulna
Long Bone, Appendicular Skeleton
Also known as: Forearm Bone, Elbow Bone
Classification
Long Bone, Appendicular Skeleton
Dimensions
In small mammals such as squirrels or rabbits, this bone typically ranges from 1 to 3 inches in length. It is long, slender, and distinctly tapered, featuring a flared upper joint structure and a much thinner shaft that ends in a small rounded point.
Key Features
Look for the unmistakable 'wrench head' shape at the top end. This hook-like crescent notch resembles an adjustable wrench or ice scoop and forms the hinge of the elbow joint. Directly above this scoop is a sturdy knob that acts as the point of the elbow. The long shaft gently curves down to a narrow point at the lower end.
Similar Bones
It can be distinguished from its companion forearm bone (the radius) by its pronounced wrench-like upper notch; the radius has a round, button-like head at its top instead. It might also be confused with a shin bone (tibia), but the shin bone lacks the deep, hook-shaped cutout at the joint end and has a broader, flatter top surface.
Across Species
The structure of this forearm bone varies widely across animal species. In highly cursorial (running) mammals like horses and deer, it fuses almost entirely with the primary arm bone (radius) to form a single rigid lever optimized for forward motion. In contrast, climbing and digging species such as cats, raccoons, and rodents retain a free, well-developed bone that allows for rotational forearm movement and dexterous paw manipulation. Marine mammals like seals and whales have shortened, flattened versions that support powerful flippers.
Evolutionary History
This bone evolved as one of the two supporting elements of the lower forelimb in early land-dwelling tetrapods over 350 million years ago. Its prominent upper projection, the hook-like elbow lever, developed to anchor powerful muscle attachments necessary for supporting body weight off the ground, propelling movement forward, and enabling specialized functions like digging, climbing, or flying.
Photography Tips
Place the specimen beside a clear ruler or coin for scale. Capture a direct side view to showcase the dramatic wrench-like crescent notch at the upper joint. Using angled side lighting will highlight the deep relief of the joint surface and the subtle ridge along the slender shaft.