Femur
Long Bone of the limb (appendicular) skeleton
Also known as: Thigh Bone
Classification
Long Bone of the limb (appendicular) skeleton
Dimensions
The longest and heaviest bone in most mammals, typically characterized by a cylinder-like shaft with a rounded sphere at the top and two large rollers at the bottom.
Key Features
The most distinctive feature is the smooth, orb-shaped head (shown in blue) that sits on a neck-like projection. Below this neck are large, bony knobs for muscle attachment. The bottom ends in two smooth, rounded pads that meet the shin bone.
Similar Bones
This bone is most often confused with the upper arm bone (humerus). You can tell them apart because the femur has a much more pronounced, distinct neck supporting its spherical head, whereas the arm bone's head sits more directly on top of the shaft. The femur is also generally much thicker and longer.
Across Species
The femur adapts to various modes of travel. In massive land animals like elephants, it is pillar-like to support weight. In aquatic mammals like whales, it has become tiny and disconnected from the rest of the skeleton. In jumping animals like frogs, it is exceptionally long and thin. The blue highlighted section is the rounded head of the bone, which fits into a socket like a ball-and-socket joint, allowed for wide ranges of movement in many mammals.
Evolutionary History
Tracing back to the first vertebrates that crawled onto land, the femur evolved from the fleshy lobe fins of prehistoric fish. Over hundreds of millions of years, it lengthened and strengthened to move the body against the force of gravity, eventually rotating beneath the body in lineages like mammals and dinosaurs to allow for efficient upright walking.
Photography Tips
To help with identification, take one photo from the front and one from the side. Place a ruler or common object like a coin next to it for scale. Ensure the spherical 'ball' at the top and the rounded pads at the bottom are clearly visible and well-lit.
Notes
What is the blue highlighted part?