Humerus
Long bone of the appendicular skeleton (limb)
Also known as: Upper Arm Bone
Classification
Long bone of the appendicular skeleton (limb)
Dimensions
In large mammals, this bone is typically thick and sturdy, roughly the length of the forearm. It features a wide, flared bottom end and a rounded, ball-like top end.
Key Features
Look for a smooth, spool-shaped joint or rounded pulleys at the bottom end and a prominent rounded ball at the top. It often has a noticeable ridge along the shaft where muscles once attached. The presence of deep grooves or pits at the bottom end, which look like a hinge for the elbow, is a classic identification marker.
Similar Bones
It can be confused with the femur (thigh bone). To tell them apart, look at the top joint: the femur usually has a distinct, narrow 'neck' leading to the ball, whereas this bone has a ball that sits more directly on the main shaft. Additionally, the bottom hinge of this bone resembles a spool or pulley, while the femur's bottom joint looks more like two smooth, rounded rollers.
Across Species
Across the animal kingdom, this bone adapts to movement needs. In horses, it is short and extremely powerful for sprinting; in whales, it is flattened and shortened into a paddle-like shape for steering through water. In birds, the bone is often hollow and light to assist with flight, while in digging animals like moles, it is incredibly wide and robust to handle the pressure of moving soil.
Evolutionary History
This bone can be traced back over 350 million years to the sturdy fins of lobe-finned fish. As vertebrates moved onto land, these fin supports evolved into the load-bearing limbs of early tetrapods. Over millions of years, the shape of the upper arm bone refined to allow for greater rotation and weight support, moving from a splayed-out lizard-like posture to the vertical alignment seen in modern mammals.
Photography Tips
Lay the bone on a neutral, solid-colored background. Take high-angle shots from the front, back, and both sides. Be sure to include a clear view of the rounded top joint and the spool-shaped bottom joint. Placing a ruler or common object like a coin next to the specimen helps viewers understand its scale.