Humerus
Long bone of the appendicular skeleton
Also known as: Upper Arm Bone
Classification
Long bone of the appendicular skeleton
Dimensions
Varies greatly by species, ranging from a few centimeters in small rodents to over a meter in large mammals like elephants. It features a sturdy central shaft flanked by a rounded top end and a wider, pulley-like lower end.
Key Features
Look for a long, mostly straight central shaft. One end features a smooth, rounded ball or bulbous shape, while the opposite lower end broadens out into distinct bumps and a smooth, grooved spool shape designed to hinge with the forearm bones.
Similar Bones
Beginners might mistake the humerus for the femur (thigh bone). You can tell them apart by looking at the lower end: the humerus features a pulley-like shape for a complex hinge joint, whereas the lower end of the femur usually features two distinct, rounded knobs designed to work with the knee.
Across Species
In humans, the humerus is built for a wide range of arm movements and shoulder rotation. In running mammals like horses and deer, it is tucked tightly against the body, shortening the upper segment to lengthen the lower stride. In swimming mammals like whales and dolphins, the humerus has dramatically shortened and widened, turning into a stout support pillar inside the flipper.
Evolutionary History
The humerus traces its ancestry back to the paired fins of ancient lobe-finned fish. Over millions of years of land-dwelling evolution, this basic fin-support structure elongated and developed specialized joint surfaces at both ends to lift the body off the ground and enable walking and running.
Photography Tips
Place the bone flat against a neutral, high-contrast background with a ruler or centimeter scale placed right alongside it. Take one photo capturing the full length from the front, and a second close-up photo focusing specifically on the lower end to show the detailed curves of the joint surface.