Ulna (Coronoid Process highlighted)
Long Bone (Appendicular Skeleton)
Also known as: Lower Arm Bone, Elbow Bone
Classification
Long Bone (Appendicular Skeleton)
Dimensions
The ulna is a long, slender bone that is typically longer than its companion, the radius. It tapers from a thick, wrench-like upper end to a very thin lower end.
Key Features
The most striking feature is the top end, which looks remarkably like a crescent wrench or a hook. The blue highlighted section is the coronoid process, a triangular ridge that acts as a front-stopper for the elbow joint. The shaft has a sharp outer ridge where a membrane attaches to the radius.
Similar Bones
The radius is most often confused with the ulna. Look for the top end: the radius has a flat, circular 'button' at its top, whereas the ulna has the large, deep 'hook' or 'wrench' shape.
Across Species
The ulna shows incredible variation across the animal kingdom. In hoofed animals like horses and deer, the ulna is often fused to the radius or reduced to a splinter to provide stability for running. In contrast, in swimming mammals like whales, it becomes short and paddle-like. Flying mammals like bats have an extremely thin, almost needle-like ulna to save weight.
Evolutionary History
Tracing back to early tetrapods, the ulna emerged as one of the two primary weight-bearing supports of the front limb. Over millions of years, the shape of the upper end evolved to lock more securely into the upper arm bone, allowing for the specialized hinge-like motion of the elbow seen in modern mammals.
Photography Tips
To identify an ulna from a photo, try to capture the large hook-shaped end from the side to show its deep curve. Take a second photo looking straight down at the top of the hook and a third showing the entire length of the bone against a ruler for scale.
Notes
What is the blue highlighted part?