Humerus
Long Bone, part of the limb (appendicular) skeleton
Also known as: Upper Arm Bone
Classification
Long Bone, part of the limb (appendicular) skeleton
Dimensions
Varies widely by species, featuring a sturdy central shaft flanked by two broader, flared ends that serve as major anchor points for muscles and joints.
Key Features
Look for a long, straight or gently curved central shaft connecting two distinctly shaped ends. One end typically features a smooth, rounded knob for ball-and-socket movement, while the opposite end fans out into pulleys and ridges designed to create a hinge joint.
Similar Bones
Can sometimes be confused with the thigh bone, but the upper arm bone is generally shorter and features a distinctively shaped shoulder attachment rather than the long, sharply angled neck found on the leg counterpart.
Across Species
In running mammals like horses and deer, this bone is tightly bound within the body wall, appearing short and heavily built. In flying birds, it is wonderfully lightweight and hollow, often reinforced with internal struts to withstand the stresses of flight. In marine mammals like whales, the bone has shortened drastically, turning into a stout, compact cylinder that helps form a rigid flipper for steering through water.
Evolutionary History
The upper arm bone traces its ancestry back to the paired fins of ancient lobe-finned fish that first explored shallow waters hundreds of millions of years ago. As vertebrates transitioned entirely to land, this single foundational bone adapted to support the full weight of the front half of the body, eventually diversifying into the wide array of leaping, digging, flying, and swimming limbs seen in modern land animals.
Photography Tips
Place a coin or ruler next to the bone for scale, shoot from directly overhead to capture the full length, and use natural sunlight coming from the side to highlight the bumps and ridges along the ends.