Ulna
Long Bone, appendicular skeleton (limb frame)
Also known as: Elbow bone, forelimb bone
Classification
Long Bone, appendicular skeleton (limb frame)
Dimensions
In small mammals like squirrels or rabbits, this delicate bone typically measures between 3 and 7 centimeters in length. It is slender and slightly curved, featuring a prominent hook-like projection at the top end.
Key Features
Look for the distinctive wrench-like notch near the upper top end, which forms the hinge of the elbow joint. Just above this notch is a strong, pointed projection that creates the sharp bump of the elbow tip. The shaft tapers down into a narrow, smooth rod ending in a tiny knob at the bottom.
Similar Bones
The ulna can easily be confused with its neighboring forelimb bone, the radius. However, the radius has a rounded, disc-shaped top end designed to pivot, whereas the ulna features a hook-shaped notch and a pronounced elbow crest. It can also be distinguished from bird wing bones by its smooth shaft, which lacks the raised feather-attachment bumps typical of avian ulnas.
Across Species
The ulna serves as an anchor for powerful arm muscles across many species. In digging animals like moles, it is extremely thick and short, acting like a heavy chisel. In birds, the ulna forms a major component of the wing, often showing tiny bumps along its ridge where flight feathers attach. In hoofed mammals like horses, the ulna fuses almost completely with its neighbor, the radius, to create a rigid, lightweight leg built purely for running.
Evolutionary History
Dating back to the earliest four-limbed land vertebrates over 350 million years ago, the pair of lower-arm bones provided stability and rotational mobility for weight-bearing limbs. Over millions of years, as different animal groups adapted to swimming, flying, running, or climbing, the ulna changed shape dramatically—sometimes expanding to support powerful clawing muscles, and other times shrinking or fusing to lighten the limb for flight or high-speed land locomotion.
Photography Tips
Lay the bone flat alongside a standard coin or scale bar to show its length and delicate scale clearly. Capture a close-up photo of the top end from a side angle to highlight the characteristic hook shape and notch of the elbow joint. Bright overhead light with soft side illumination helps bring out the subtle textures and contours along the shaft.