Femur
Long Bone; part of the limb (appendicular) skeleton.
Also known as: Thigh Bone
Classification
Long Bone; part of the limb (appendicular) skeleton.
Dimensions
Generally the longest, heaviest, and strongest bone in a land vertebrate's body. It features a long cylindrical shaft and two uniquely shaped ends for the hip and knee joints.
Key Features
The most defining feature is the ball-shaped 'head' at the top end, which looks like a smooth door knob sitting on a short neck. Conversely, the bottom end features two large, smooth rollers that resemble knuckles. Along the back of the central shaft, there is a distinct vertical ridge that serves as an anchor for powerful leg muscles.
Similar Bones
It can be confused with the Humerus (upper arm bone). However, the Humerus has a much shorter neck at the top ball and the lower end is more spool-shaped rather than having two distinct large 'knuckles.' The femur is also typically much thicker and heavier than the Humerus.
Across Species
The femur is a master of adaptation across the animal kingdom. In heavy mammals like elephants, it is thick and pillar-like to support immense weight. In contrast, many birds have femora that are hollow and air-filled to aid in flight. In marine mammals like whales, the femur has become significantly shortened and may even be detached from the rest of the skeleton, a remnant of their land-dwelling ancestors. Horses have a prominent third bump on the shaft for massive muscle attachment, which helps power their gallop.
Evolutionary History
The femur first appeared in early lobe-finned fish, which used these sturdy structures to pivot their fins against the floor of shallow waters. As life moved onto land, the femur rotated from a sprawling position (like that seen in modern crocodiles) to a vertical position directly under the body in dinosaurs and mammals, allowing for much more efficient and rapid movement over long distances.
Photography Tips
Lay the bone on a neutral, non-reflective surface. Take one photo from directly above to show the full length. Take a second close-up photo of the ball-shaped head and a third photo of the knuckle-like rollers at the bottom. Including a ruler or a common object like a coin helps viewers understand its true size.