Femur
Long Bone; part of the appendicular (limb) skeleton
Also known as: Thigh Bone
Classification
Long Bone; part of the appendicular (limb) skeleton
Dimensions
The femur is generally the largest, densest, and heaviest bone in most vertebrate bodies. It is much longer than it is wide, consisting of a stout central shaft with rounded, complex joints at each end.
Key Features
The most distinctive part is the 'head,' a smooth ball-shaped end that fits into a socket. Below this is a narrower neck and several large, rugged bumps where heavy muscles attach. At the bottom end, two large, rounded rolling surfaces look somewhat like a spool of thread or a set of knuckles, which allow it to hinge at the knee.
Similar Bones
The femur can be confused with the humerus (upper arm bone). However, the femur is typically much heavier and has a more pronounced neck and ball at the top. The lower end of a femur has rounded rolling surfaces for the knee, whereas the humerus has a more complex, spool-shaped joint for the elbow.
Across Species
In mammals, the femur varies widely according to lifestyle. In fast-running ungulates like horses, it is relatively short and thick to provide leverage for heavy muscle attachments. In contrast, in aquatic mammals like whales, it may be reduced to a tiny, floating vestigial bone. In birds, the femur is often short and tucked against the body, while the lower leg bones are much longer, which keeps their heavy flight muscles close to their center of gravity.
Evolutionary History
The femur first appeared in early lobe-finned fish during the Devonian period, acting as a supporting element for pelvic fins. As animals transitioned to land, this bone evolved into a powerful vertical pillar capable of lifting the body off the ground. Over millions of years, the shape of the upper end changed from a straight attachment to a curved 'neck' and 'head' in mammals, allowing for more efficient striding and better weight distribution.
Photography Tips
To help with identification, place a common object like a coin or a pen next to the bone for size reference. Take photos from a direct top-down view and from the side. Capturing the ends of the bone where the joints are is crucial, as the unique shapes of the connection points are the best clues for tells one bone from another.