Humerus
Long bone of the appendicular skeleton
Also known as: Upper Arm Bone
Classification
Long bone of the appendicular skeleton
Dimensions
Varies widely by species, ranging from a few inches in small mammals to over two feet in large ungulates. It typically features a sturdy central shaft flanked by distinctly shaped, bulbous ends.
Key Features
Look for a smooth, rounded knob at one end designed for a ball-and-socket shoulder joint, contrasted with a spool-shaped or grooved surface at the opposite end for the elbow hinge. The central shaft is usually stout with prominent rough patches where powerful shoulder and chest muscles attach.
Similar Bones
Can be confused with the thigh bone (femur), but differs by having a distinct elbow-connecting spool at the bottom end rather than a second rounded hip-joint ball. It can also resemble the shin bone, which generally lacks the single prominent rounded shoulder ball at the top.
Across Species
In running animals like horses and deer, the upper arm bone is tucked closely against the body wall and built for heavy load-bearing and forward propulsion. In burrowing animals like moles, it transforms into a short, exceptionally broad anchor for massive digging muscles. In soaring birds, it is lightweight and often hollow to save weight for flight.
Evolutionary History
The basic design of this upper limb bone traces back hundreds of millions of years to the earliest land-dwelling four-legged animals. Over deep time, its joints and muscle attachment ridges have shifted to accommodate crawling, walking, running, swimming, or flying.
Photography Tips
Clear away surrounding leaf litter to expose the full outline. Place a coin or ruler next to the bone for scale. Take one straight-down photo showing the entire length, and two additional close-up shots focusing separately on the upper and lower ends to capture the unique joint shapes.