Humerus
Long Bone belonging to the appendicular skeleton
Also known as: Upper Arm Bone
Classification
Long Bone belonging to the appendicular skeleton
Dimensions
Length varies greatly by species, ranging from a few inches in small mammals to several feet in large quadrupeds, generally featuring an elongated central shaft with expanded, rounded ends.
Key Features
Look for a long, roughly cylindrical central shaft that transitions into wider, bulbous or blocky ends. The upper end typically features a smooth, rounded knob for joint movement, while the lower end has a pulley-like shape for connecting to the lower arm bones.
Similar Bones
Beginners often confuse the humerus with the femur or the large lower leg bones. You can tell the humerus apart by looking at the specific shape of its ends—such as the spherical ball at the upper end and the characteristic ridges and grooves at the lower end designed for elbow movement.
Across Species
In running mammals like horses, the humerus is short and heavily muscled, tucked close to the body. In flying birds, it is lightweight and hollow, acting as an anchor for powerful flight muscles. In swimming marine mammals like whales, the humerus has become very short and stout, helping form a sturdy paddle.
Evolutionary History
The humerus traces its ancestry back to the paired fins of ancient lobe-finned fish. Over hundreds of millions of years, as vertebrates transitioned to land, this fin bone transformed into a robust lever capable of supporting the animal's weight and facilitating locomotion.
Photography Tips
Place a common object like a coin or ruler next to the bone for scale. Take your primary photo from directly overhead with bright, even daylight to highlight the contours and texture of the surface. Additional angled shots focusing on both ends will help capture the joint surfaces clearly.