Humerus
Long Bone of the limb (appendicular) skeleton
Also known as: Upper Arm Bone
Classification
Long Bone of the limb (appendicular) skeleton
Dimensions
The humerus is typically a robust, cylindrical bone. In medium-to-large mammals like deer, it often ranges from 8 to 12 inches in length. It is generally thicker and shorter than the bones of the lower limb, providing a sturdy foundation for the arm or front leg.
Key Features
To identify a humerus, look for a rounded 'ball' at the top end that fits into the shoulder. The middle shaft is slightly twisted or spiraled. The bottom end, where it meets the elbow, features a distinct spool-shaped surface and a deep hollow on the back side where the lower arm bone locks in.
Similar Bones
The humerus is most often confused with the femur (thigh bone). You can distinguish them by looking at the top: the femur has a very prominent, distinct 'neck' supporting its ball-shaped head, whereas the ball of the humerus sits more directly atop the main shaft. The humerus bottom also features a spool-like surface rather than the two large rounded rockers found on the femur.
Across Species
In the natural world, the humerus adapts to an animal's lifestyle. In birds, it is often hollow and light to facilitate flight. In swimming mammals like whales, it becomes short and flattened to form the base of a powerful flipper. In running animals like deer or horses, it is tucked high up against the chest, acting as a powerful lever for swift movement.
Evolutionary History
The humerus first appeared in the lobe-finned fish that eventually transitioned to land millions of years ago. Over deep time, it evolved from a broad, flat fin-support into the elongated lever we see in modern land animals, allowing for complex terrestrial movement like crawling, climbing, and running.
Photography Tips
Lay the bone on a flat surface next to a common object like a coin or a ruler for scale. Take one photo from directly above to show the overall shape, and another focused on the joint ends (the 'elbow' and 'shoulder') which contain the most unique identification markers.