Humerus
Long Bone (appendicular skeleton)
Also known as: Upper arm bone, funny bone
Classification
Long Bone (appendicular skeleton)
Dimensions
This is typically the longest and largest bone of the upper limb. It features a long cylindrical shaft with a rounded head at the top and a wider, flatter, more complexly shaped area at the bottom.
Key Features
Look for the distinctive 'sand-clock' or pulley shape at the lower end, which creates a hinge for the elbow. Along the mid-section of the shaft, there is often a rough, raised triangular patch where shoulder muscles attach. The bottom end is characterized by two noticeable bumps on the sides and a deep hollow in the back where the forearm bone rests when the arm is straightened.
Similar Bones
The humerus is often confused with the femur (thigh bone). To tell them apart, look at the top: the femur has a distinct 'neck' and a much more pronounced ball-shaped head, whereas the humerus has a more rounded head and a flatter, more flared bottom end compared to the rounder knobs of the knee.
Across Species
In the natural world, the humerus shows incredible diversity. In bats, it is thin and lightweight for flight, while in whales and seals, it is shortened and thickened into a stout paddle-like shape for steering through water. In burrowing animals like moles, it is exceptionally wide and rugged to anchor the massive muscles needed for digging through soil.
Evolutionary History
The humerus traces its ancestry back over 350 million years to the lobe-finned fish. As vertebrates moved from water to land, this bone transitioned from a flat fin support into a load-bearing column. Over millions of years, the bottom end evolved complex pulley-like surfaces to allow for the sophisticated rotational movements seen in modern climbing and tool-using mammals.
Photography Tips
Capture the bone from multiple angles, particularly focusing on the bottom hinge-like area. Use flat, indirect natural lighting to highlight the subtle ridges on the shaft. Always include a common object like a coin or a ruler for scale, and take photos of both the front and the back surfaces.