Humerus
Long Bone of the limb (appendicular) skeleton
Also known as: Upper Arm Bone
Classification
Long Bone of the limb (appendicular) skeleton
Dimensions
Typically long and sturdy, the humerus features a rounded ball at the top end and a flared, hinge-like structure at the bottom end. In mammals, the length of the shaft is generally several times its width, though proportions vary by species.
Key Features
Look for a large, rounded head at the top that fits into a socket. The middle section (the shaft) often shows a slight twist. The bottom end is distinctive for its pulley-shaped surface and a deep pit in the back that allows the lower arm bone to lock in place when the limb is straightened. Depending on the animal, there may be a prominent ridge on the outer side for muscle attachment.
Similar Bones
This bone is most commonly confused with the femur (thigh bone). To tell them apart, look at the top: the humerus has a rounded head that sits directly on the column, while the femur usually has a distinct 'neck' that offsets the ball from the shaft. Additionally, the bottom of the humerus looks like a single pulley, whereas the femur has two rounded lobes like a knuckle.
Across Species
Across the animal kingdom, the humerus is a master of adaptation. In moles, it is short, wide, and incredibly powerful for digging. In birds, the bone is often hollow (pneumatic) to reduce weight for flight, while in whales and seals, it has evolved into a short, flattened block that supports a paddle-like flipper for steering through water. In hoofed animals like deer or cows, it is thick and robust to support significant weight during movement.
Evolutionary History
The humerus traces its lineage back to the lobe-finned fish of the Devonian period, over 300 million years ago. As early vertebrates transitioned from water to land, this single upper bone became the foundational lever of the front limb. Over eras, it has shifted from a sprawling posture to an upright, weight-bearing column in modern mammals, reflecting a journey from swimming to crawling and finally to running and climbing.
Photography Tips
To help with identification, take a clear photo of the top rounded head and another of the bottom pulley-like end. A side-profile view showing the curve of the shaft is also helpful. Include a common object like a coin or ruler in the frame to show the scale of the specimen.