Humerus
Long bone of the appendicular skeleton (limbs)
Also known as: Upper arm bone, Funny bone
Classification
Long bone of the appendicular skeleton (limbs)
Dimensions
The longest and largest bone of the upper limb, typically characterized by a cylindrical shaft with a rounded head at the top and a flared, complex base at the bottom.
Key Features
The bottom end is identifiable by its distinctive bowtie or spool-like shape, which fits into the forearm. Look for the two prominent bumps on the sides; the inner bump is usually larger and more flared, while the outer side is more rounded. Directly above the bottom joint is a deep pit that accommodates the elbow bone when the limb is straightened.
Similar Bones
The femur (thigh bone) is the most common look-alike, but it is typically much larger and heavier. To tell them apart, look at the bottom: the femur has two large, rounded rollers for the knee, whereas the humerus has a more complex, interlocking spool-like surface for the elbow.
Across Species
In birds, this bone is often hollow and light to assist with flight. In horses and other ungulates, the humerus is relatively short and sturdy to provide leverage for high-speed running. In aquatic mammals like whales, it is significantly shortened and compressed into a stout, blocky shape to support powerful flippers.
Evolutionary History
The humerus emerged from the fleshy lobe-fins of ancient sarcopterygian fish. Over millions of years, as vertebrates transitioned to life on land, this bone elongated and developed specialized hinge-like features at the bottom to support the weight of the body and allow for a wide range of terrestrial movement.
Photography Tips
Capture the flared bottom end from the front and back to show the depth of the central pit. Use overhead lighting to highlight the ridges and curves, and always place a common object like a coin or a ruler nearby to provide a sense of scale for the viewer.