Fibula
Long bone, appendicular skeleton (part of the lower limb)
Also known as: Calf bone, outer shin bone
Classification
Long bone, appendicular skeleton (part of the lower limb)
Dimensions
This is a remarkably long and slender bone, often resembling a flute or a thin stick. It typically spans the length of the lower leg, characterized by a length that is many times its width.
Key Features
The fibula is easily recognized by its extremely narrow, straight shaft and its two distinct ends. The top end has a slightly rounded, knob-like head that sits just below the knee, while the bottom end flares out into a flattened, arrowhead-shaped bump that forms the outer part of the ankle. It lacks the heavy, triangular cross-section of the main shin bone.
Similar Bones
The fibula is most often confused with the radius (the outer bone of the forearm). You can tell them apart because the fibula is generally much longer and thinner, and its bottom end forms a distinct ankle flare, whereas the radius has a flat, disc-like top for rotating the elbow.
Across Species
In many mammals, the fibula is a distinct, separate bone that allows for slight rotation and muscle attachment in the lower leg. However, in fleet-footed animals like horses and deer, it is often reduced to a tiny splinter or fused entirely to the larger shin bone to stabilize the leg for running. In birds, it is a thin, needle-like bone that doesn't even reach the ankle, whereas in climbing animals, it remains robust to support varied foot movements.
Evolutionary History
Tracing back to early land-dwelling tetrapods, the fibula was originally a sturdy, weight-bearing partner to the tibia. As modern land animals evolved specialized ways of moving, the tibia took over the primary job of supporting body weight, allowing the fibula to become thinner, more flexible, or in some lineages, nearly disappear as the limb specialized for high-speed travel or specialized swimming.
Photography Tips
Lay the bone on a plain, dark background to highlight its white color. Take one photo of the entire length with a ruler or common object like a coin for scale, and then take close-up shots of both ends, as the shape of these knobs is the most important clue for identification.