Femur
Long bone; part of the appendicular (limb) skeleton.
Also known as: Thigh bone
Classification
Long bone; part of the appendicular (limb) skeleton.
Dimensions
Generally the longest, heaviest, and strongest bone in the body, featuring a long cylindrical shaft and rounded ends for joint movement.
Key Features
Identifiable by its large, distinct ball-shaped head at the top that fits into the hip. The bottom end features two large, smooth rounded bumps for the knee. The blue highlighted area in the image covers the flat, triangular surface just above the knee on the back of the bone, known as the popliteal surface.
Similar Bones
Often confused with the humerus (upper arm bone). You can tell them apart because the femur has a much more pronounced 'neck' connecting the ball-head to the shaft, and the lower end of the femur has two large, matching rounded lobes, whereas the humerus has a more complex, pulley-shaped bottom.
Across Species
The femur is a hallmark of four-legged and two-legged land vertebrates. In fast-running mammals like horses, the upper shaft has a prominent third bump for massive muscle attachments. In whales, the femur is often tiny or missing entirely, a remnant of their land-dwelling ancestors. Birds have a relatively short, thick femur that stays mostly tucked within their body feathers, acting as a hidden lever for their legs.
Evolutionary History
This bone emerged as vertebrates moved from water onto land. Ancient lobe-finned fish had sturdy fin bones that eventually transitioned into the femur seen in early amphibians. Over millions of years, the bone shifted from a sprawling position out to the sides to a more upright position tucked directly under the body, allowing for much more efficient walking and running in dinosaurs and mammals.
Photography Tips
To get a clear identification, photograph the bone lying flat with a ruler next to it for scale. Take one photo of the whole bone, and then close-ups of the ball at the top and the rounded hinges at the bottom from both the front and back views.
Notes
What is the blue highlighted part?