Femur
Long bone of the appendicular skeleton
Also known as: Thigh bone
Classification
Long bone of the appendicular skeleton
Dimensions
As the longest and strongest bone in the body, the femur typically accounts for about one-fourth of a person's overall height. It features a long, slightly curved central shaft with distinct, enlarged ends designed to connect the hip to the lower leg.
Key Features
The most striking features are the smooth, rounded ball at the upper end designed to fit into a socket, a prominent neck connecting that ball to the main shaft, and two large, smooth, roller-like knobs at the lower end that create a pulley system for the knee.
Similar Bones
Beginners sometimes confuse the upper arm bone, the humerus, with the femur. While both are long bones with a ball-like top end, the femur's ball is much more spherical and sits on a distinct, angled neck, whereas the humerus has a flatter, less pronounced ball. The lower end of the femur also features two distinct, parallel curved knobs for the knee, unlike the broader, flatter connecting surfaces found at the elbow end of the humerus.
Across Species
The femur is found in all tetrapods, but its shape varies wildly depending on lifestyle. In running mammals like horses and cheetahs, the upper end features heavy, specialized anchor points for powerful leg-swinging muscles. Whales and dolphins have lost external hind limbs entirely, leaving behind only tiny, floating remnants of a femur buried deep within their muscles. Birds have a remarkably short, stout femur that remains mostly hidden close to the body, as their primary walking and running power is concentrated further down the leg.
Evolutionary History
The femur originated over 400 million years ago from the sturdy limb bones of ancient lobe-finned fish that first crept onto land. Over millions of years, as vertebrates shifted from sprawling, lizard-like movements to upright, pillar-like stances beneath their bodies, the femur straightened and developed its characteristic rounded upper ball joint to swivel smoothly within the hip socket.
Photography Tips
To get a great identification photo, place the bone on a neutral, solid-colored background with good, even lighting from the side to highlight the bumps and curves. Take one photo showing the entire length from the top and bottom views, and a close-up shot of each end. Always include a small coin or ruler next to the bone to provide a clear sense of scale.