Humerus
Long Bone, part of the appendicular skeleton
Also known as: Upper Arm Bone
Classification
Long Bone, part of the appendicular skeleton
Dimensions
Varies widely depending on species, generally featuring a long central shaft with expanded rounded ends designed for joint attachment.
Key Features
Characterized by a smooth, roughly cylindrical central shaft flanked by distinct ends. One end features a smooth, rounded ball shape for resting in a shoulder socket, while the opposite end flares out into a wider, spool-shaped hinge surface for connecting to the lower arm bones.
Similar Bones
Can sometimes be confused with the upper leg bone (femur). You can tell them apart by looking at the top end: the upper arm bone typically features a more spherical ball attached at a distinct angle via a short neck, whereas the upper leg bone's ball is usually set more prominently off to the side with a clearly defined, elongated neck region.
Across Species
The upper arm bone is a masterclass in evolutionary versatility. In humans, it allows for a wide circle of shoulder movement and tool use. In running mammals like horses and dogs, it is tucked closely into the body, shortened and aligned strictly for forward-and-backward stride efficiency. In birds, it forms the sturdy anchor for the wing muscles, while in swimming marine mammals, it has been compressed and shortened into a paddle-like structural block.
Evolutionary History
Tracing its roots back to the paired fins of ancient lobe-finned fish, this bone transitioned onto land alongside the earliest tetrapods. Over hundreds of millions of years, the single robust ancestral fin bone elongated and refined into the distinct upper arm structure seen in modern land-dwellers.
Photography Tips
To get a great reference photo, place the bone on a neutral, solid-colored background with a common coin or ruler next to it for scale. Take one shot looking straight down at the top surface, and another shot from the side to capture the natural curves and the shape of both ends.