Humerus
Long bone of the limb (appendicular) skeleton
Also known as: Upper Arm Bone
Classification
Long bone of the limb (appendicular) skeleton
Dimensions
In large mammals, this bone is typically thick and sturdy. It is longer than it is wide, with heavy, flared ends meant for muscle and joint connection.
Key Features
Look for a long, cylindrical shaft that widens significantly at the bottom. The lower end often features a distinct pulley-like shape or smooth, rounded surfaces where it meets the lower limb bones. There is typically a noticeable pit or depression just above the joint surface at the bottom, which allows for a wide range of movement.
Similar Bones
The femur (thigh bone) is the most common look-alike. You can tell them apart because the humerus usually has a more flattened, pulley-shaped lower end, whereas the femur's lower end has two very distinct, large rounded 'rollers' and a much more prominent ball-shaped head at the top.
Across Species
The humerus is a master of adaptation in the animal kingdom. In birds, it is often hollow (pneumatic) to reduce weight for flight. In moles, it is short, broad, and incredibly powerful for digging. In whales and dolphins, the humerus is shortened and flattened to form a rigid support for a swimming flipper. Large grazing animals like cows or horses have a massive, columnar humerus to support their significant body weight during movement.
Evolutionary History
This bone can be traced back over 360 million years to the first lobe-finned fish that began hauling themselves onto land. What started as a stout fin-support in creatures like Tiktaalik eventually lengthened and strengthened to facilitate upright walking and swinging limbs. It has remained a core component of the limb structure in nearly all amphibians, reptiles, birds, and mammals throughout deep time.
Photography Tips
Lay the bone on a flat, neutral surface and place a common object like a coin or a ruler next to it for scale. Take one photo from directly above to show the overall shape, and another a bit closer to the flared ends to capture the details of the joint surfaces.