Femur
A Long Bone of the limb (appendicular) skeleton.
Also known as: Thigh Bone
Classification
A Long Bone of the limb (appendicular) skeleton.
Dimensions
The femur is generally the largest and strongest bone in a mammal's body. It consists of a long, cylindrical shaft with a rounded head at the top and a widened base at the bottom.
Key Features
The most distinctive feature is the smooth, ball-shaped 'head' at the top, which resembles a trailer hitch. Next to this ball is a prominent, often rugged bump called the trochanter. The shaft is thick and usually has a slight curve, ending in two large, rounded rockers at the bottom that form the top of the knee.
Similar Bones
The humerus (upper arm bone) is often confused with the femur. To tell them apart, look at the top: the femur has a distinct, narrowed 'neck' connecting the ball-head to the shaft, whereas the head of the humerus sits more directly on the top of the bone without a long neck.
Across Species
The femur varies wildly based on an animal's locomotion. In cursorial (running) animals like deer or horses, it is long and straight to maximize stride. In contrast, massive animals like elephants have thick, pillar-like femurs to support immense weight. In whales, the femur has evolved into a tiny, vestigial remnant buried deep within the blubber, no longer attached to the rest of the skeleton.
Evolutionary History
The femur's origin traces back to the lobe-finned fish of the Devonian period. As vertebrates transitioned to land, this bone strengthened and changed its orientation from sticking out sideways (like in a lizard) to resting directly beneath the body in mammals and birds, allowing for more efficient movement and the support of greater body mass.
Photography Tips
To help with identification, place a ruler or a common object like a coin next to the bone for scale. Take one photo looking directly at the ball-shaped head, one from the side to show the shaft's curve, and one of the flattened or flared lower end.