Ulna
Long bone, part of the appendicular skeleton (limb)
Also known as: Inner Forearm Bone
Classification
Long bone, part of the appendicular skeleton (limb)
Dimensions
Typically ranges from 20 to 30 centimeters in length for an adult human, featuring a slender shaft that tapers from a bulky, wrench-shaped upper end down to a small, button-like lower tip.
Key Features
Look for the prominent wrench- or C-shaped hook at the upper end, designed to cradle the end of the upper arm bone. The long shaft has a gentle curve and sharp ridges that anchor the muscles used for gripping and twisting the hand.
Similar Bones
Beginners frequently confuse the ulna with its forearm companion, the radius. While the ulna features a distinct hook-like grip at one end, the radius is generally more symmetrical and ends in a smooth, shallow cup shape. It can also be mistaken for the lower leg bone (fibula), which lacks the large gripping hook entirely.
Across Species
In many four-legged mammals like dogs and horses, the ulna and its companion forearm bone are fused or partially reduced to prevent twisting and specialize strictly in forward-and-back running strides. In contrast, primates and many climbing mammals retain two distinct, highly mobile forearm bones that allow the hand to rotate freely.
Evolutionary History
The ulna traces its deep ancestry back to the paired fin bones of ancient lobe-finned fish. Over hundreds of millions of years of land-dwelling evolution, this skeletal element transitioned from supporting a sprawling, push-up-style swimming or crawling limb into a vertical support strut capable of bearing weight and granting rotational agility.
Photography Tips
Place the bone against a solid, contrasting background next to a standard coin or ruler for scale. Capture a clear top-down view showing the entire length, followed by close-up shots focusing on the distinct hooked shape of the upper end and the narrower lower tip.
Notes
Human ulna?