Humerus
Long Bone of the appendicular skeleton
Also known as: Upper Arm Bone
Classification
Long Bone of the appendicular skeleton
Dimensions
Proportions vary wildly depending on the species, featuring a sturdy central shaft flanked by expanded, knobby ends designed for muscle attachment.
Key Features
Look for a smooth, tubular central shaft that flares out into distinct articular surfaces at both ends. One end typically features a rounded, ball-like knob designed to fit into a socket, while the opposite end fans out into a pulley-like structure for the elbow joint.
Similar Bones
Can be easily confused with the femur or thigh bone. You can tell the humerus apart by looking at the specific shape of the upper end: a humerus usually has a more hemispherical ball offset at an angle, whereas a femur features a distinct spherical head attached by a defined neck.
Across Species
In birds, this bone is lightweight and often hollow to assist with flight, whereas in swimming mammals like whales, it becomes short and stout. In running mammals like horses, it is heavily tucked into the shoulder muscles, making the upper arm quite short relative to the lower leg.
Evolutionary History
The humerus traces its lineage directly back to the paired fins of ancient lobe-finned fishes. Over hundreds of millions of years, as vertebrates transitioned to land, this fin bone elongated and reinforced itself to support the weight of the body against gravity.
Photography Tips
Place the bone on a neutral, non-glare surface with a coin or ruler next to it for scale. Take one straight-down photo showing the entire length, and a close-up shot focusing specifically on the complex joint ends where the most identifying bumps and curves reside.