Femur
Long bone of the appendicular skeleton
Also known as: Thigh bone
Classification
Long bone of the appendicular skeleton
Dimensions
Varies widely by species, from a fraction of an inch in small mammals to several feet in large quadrupeds, characterized by a long central shaft and expanded upper and lower ends.
Key Features
Features a smooth, rounded ball at the upper end that fits into the hip socket, a long straight or gently curved central shaft, and two wide, knobby bumps at the lower end that help form the knee joint.
Similar Bones
Can be confused with the humerus (upper arm bone). The femur can usually be distinguished by the offset, spherical ball at the upper end that connects sharply at an angle to the shaft, whereas the humerus generally has a more centered and less dramatically offset head.
Across Species
In running mammals like horses and deer, the femur is tucked high up near the body and heavily muscled, functioning like a high-powered piston. In cetaceans like whales, the femur is reduced to a tiny, floating vestigial nodule hidden deep inside the body wall, reflecting an aquatic lifestyle where hind limbs are no longer needed for walking.
Evolutionary History
The femur traces its ancestry back to the earliest land-dwelling tetrapods over 350 million years ago, evolving from the sturdy fin bones of ancient lobe-finned fish to support the weight of the body against gravity during terrestrial locomotion.
Photography Tips
Place the bone on a neutral, non-reflective background with a ruler or coin next to it for scale. Take one straight-on shot showing the entire length, and a separate close-up shot focusing specifically on the rounded ball at the upper end and the wide bumps at the lower end.