Femur
Long bone; part of the limb (appendicular) skeleton.
Also known as: Thigh bone, upper leg bone
Classification
Long bone; part of the limb (appendicular) skeleton.
Dimensions
The long bone of the upper leg, typically the sturdiest and heaviest bone in the vertebrate body. It usually features a distinct rounded head at the top and a widened base at the bottom.
Key Features
Look for a prominent, smooth, ball-shaped knob at the top end (the head) offset by a short 'neck.' The shaft is usually straight or slightly curved, ending in two smooth, rounded rollers at the bottom that form the top of the knee.
Similar Bones
It is often confused with the humerus (upper arm bone). However, the femur typically has a more pronounced ball-shaped head at the top and lacks the deep 'spool' shape found at the bottom of many humerus bones. The femur's head also sticks out to the side more noticeably on a distinct neck.
Across Species
In many small mammals like rodents, the femur is remarkably short compared to the lower leg bones, allowing for a coiled posture ready for springy movement. In birds, the femur is tucked close to the body and often hidden by feathers, while in marine mammals like whales, it has shrunk to a tiny vestigial nub deep within the body wall as they lost their hind limbs.
Evolutionary History
The femur emerged as life moved onto land over 350 million years ago. It began as a short, stout element in sprawling early tetrapods and evolved to become a vertical pillar in dinosaurs and mammals, supporting tremendous weight and enabling efficient long-distance travel across land.
Photography Tips
Lay the bone on a solid, neutral-colored background. Take photos from the front, back, and especially of both ends looking straight down at the joints. Placing a coin or ruler next to the bone helps viewers understand the size of the animal it came from.