Humerus
Long bone, part of the limb (appendicular) skeleton.
Also known as: Upper Arm Bone
Classification
Long bone, part of the limb (appendicular) skeleton.
Dimensions
This is typically a long, sturdy bone. It is generally thicker at the top than at the bottom, with a rounded head that fits into the shoulder socket. It is the longest bone of the upper limb.
Key Features
The most identifiable feature is the smooth, rounded ball at the top end. Just below this head, you will notice a series of bumps where muscles once attached. The long shaft is usually straight or slightly curved, and the bottom end expands into a wide, flatter area that resembles a spool or a pulley where it meets the lower arm.
Similar Bones
The thigh bone (femur) is the most common look-alike. You can tell them apart because the thigh bone has a very distinct, long 'neck' that offsets the rounded head, whereas the rounded head of the upper arm bone sits more directly on top of the shaft. Additionally, the bottom end of the thigh bone has two large, rounded knobs, whereas the upper arm bone looks more like a single twisted reel.
Across Species
The upper arm bone is remarkably adaptable across the animal kingdom. In whales and seals, it has become short and thick to form the core of a powerful paddle or flipper. In birds, the bone is often hollow and light to allow for flight, while in horses and other hoofed animals, it is shorter and tucked high up against the body, protected by thick muscle to provide power for galloping.
Evolutionary History
This bone can be traced back over 360 million years to the early tetrapods that first crawled onto land. In those early creatures, the upper arm bone was short and wide, acting as a sturdy pillar for weight-bearing. Over deep time, as animals adapted to diverse environments, this bone lengthened and refined its joints to allow for an incredible range of motion, from climbing to swimming to flying.
Photography Tips
To get the best identification photo, lay the bone on a flat surface with a neutral background. Take one photo from the front to show the full length and the shape of the joints. Take a second, close-up photo of the rounded top end from above, and a third from the bottom end. Including a ruler or common object like a coin helps viewers understand the size.