Humerus
Long Bone; part of the limb (appendicular) skeleton.
Also known as: Upper Arm Bone, Wing Bone, Funny Bone
Classification
Long Bone; part of the limb (appendicular) skeleton.
Dimensions
Generally cylindrical with flared ends. In most mammals, it is roughly one-fifth to one-fourth the length of the total limb, featuring a distinctive rounded ball at the top and a spool-like shape at the bottom.
Key Features
Look for a smooth, rounded 'ball' at the top end that fits into the shoulder. The shaft usually has a slight spiral twist or a raised ridge. The bottom end is characterized by a spool-shaped surface for the elbow joint and a deep scoop or hollow in the back where the lower arm bone locks in when straightened.
Similar Bones
The femur (thigh bone) is the most common confusion. To tell them apart, look at the top: the humerus has a ball that sits directly on top of the shaft, whereas the femur has a distinct 'neck' that offsets the ball to the side. Additionally, the humerus has a spool-like bottom, while the femur has two large, rounded rockers.
Across Species
The humerus is a versatile evolutionary tool. In birds, it is often hollow and air-filled to aid flight. In digging animals like moles, it is extremely short, wide, and powerful for burrowing. In swimming mammals like whales, it has flattened into a short, sturdy slab that acts as the internal anchor for a flipper, while in long-limbed hunters like gazelles, it is built for speed and efficient stride.
Evolutionary History
This bone first appeared in lobe-finned fish as a support for pectoral fins. As vertebrates transition to life on land, the humerus rotated and strengthened to support weight against gravity. Over millions of years, it has been sculpted by natural selection to meet the specific mechanical needs of climbing, flying, running, or swimming.
Photography Tips
Lay the bone on a flat surface with a coin or ruler next to it for size reference. Take three main shots: one from the front, one from the side to show the curve, and a top-down view of the rounded ball end. Soft, natural light helps highlight the ridges and textures without creating harsh shadows.