Humerus
Long Bone; part of the appendicular (limb) skeleton.
Also known as: Upper Arm Bone, Wing Bone
Classification
Long Bone; part of the appendicular (limb) skeleton.
Dimensions
Generally twice as long as it is wide, with bulky, rounded ends and a relatively straight central shaft. In many mammals, the length of this bone roughly matches the length of the shoulder blade.
Key Features
Look for a prominent, smooth rounded 'ball' at the top end that fits into the shoulder. The bottom end usually features a spool-shaped surface that looks like a miniature pulley, which allows the elbow to bend and straighten. The shaft often has a slight twist or a raised ridge where muscles once attached.
Similar Bones
It is most often confused with the femur (thigh bone). To tell them apart, look at the top: the femur has a distinct 'neck' that sticks out to the side before the round ball, whereas the humerus ball usually sits more directly on top of the shaft. Additionally, the bottom of the humerus looks like a single pulley, while the femur has two distinct rounded bumps.
Across Species
The humerus is a master of adaptation. In birds, it is often hollow and light to aid in flight, while in digging animals like moles, it is extremely short, broad, and powerful to act like a shovel. In whales, it has become a flattened, paddle-like bone within their flippers, and in humans, it allows for the wide range of shoulder rotation needed for throwing and reaching.
Evolutionary History
This bone can be traced back over 350 million years to the first lobe-finned fish that began hauling themselves onto land. Its basic design—a single upper limb bone connecting to two lower limb bones—is a fundamental blueprint shared by almost all land-dwelling vertebrates, from frogs to elephants.
Photography Tips
Lay the bone on a flat, neutral-colored surface. Take one photo from directly above to show the full length, and then take close-up shots of both ends. Natural, side-entry lighting is best to show the subtle ridges and textures on the shaft. Always include a common object like a coin or a ruler for scale.