Femur
Long Bone; part of the limb (appendicular) skeleton
Also known as: Thigh Bone
Classification
Long Bone; part of the limb (appendicular) skeleton
Dimensions
In many mammals, this is the longest and heaviest bone in the body, typically featuring a specialized ball-shaped head at the top and a cylindrical shaft.
Key Features
The most defining feature is the rounded 'ball' or knob at the top end that fits into the hip socket. Below this is a distinct 'neck' followed by a long, straight or slightly curved shaft. The bottom end usually features two large, rounded bumps that help form the knee joint.
Similar Bones
The upper arm bone (humerus) is most often confused with the thigh bone. To tell them apart, look at the top: the thigh bone usually has a more pronounced 'neck' connecting the ball-top to the shaft, whereas the upper arm bone's rounded top sits more flush against the shaft.
Across Species
The thigh bone shows incredible diversity across the animal kingdom. In birds, the thigh bone is often quite short and tucked up against the body, while the long 'leg' we see is actually made of other bones. In heavy animals like elephants, the thigh bone is thick and pillar-like to support massive weight. Aquatic mammals like whales have highly reduced thigh bones that are no longer attached to the spine, reflecting their transition from land to sea.
Evolutionary History
This bone can be traced back to the early four-legged land dwellers (tetrapods) that emerged from the water over 350 million years ago. Originally broad and sprawling to the sides, it gradually tucked beneath the body in many lineages to allow for more efficient, upright walking and running.
Photography Tips
Lay the bone against a flat, neutral background. Take one photo from directly above to show the overall length, and another close-up of the top end (the ball). Including a common object like a coin or a ruler in the frame helps others understand the actual size.