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 by species, characterized by a long, sturdy central shaft with a rounded ball shape at the upper end and a spool-shaped connecting area at the lower end.
Key Features
Look for a smooth, tubular central shaft flanked by a distinct rounded head at one end and a broad, notched connection surface at the opposite end. It features subtle ridges along the shaft where powerful shoulder and chest muscles anchor.
Similar Bones
Can be mistaken for the femur, the main upper leg bone. The humerus can usually be told apart by the ball shape at the top being smaller and offset at a sharper angle compared to the larger, perfectly spherical ball atop the femur.
Across Species
In tetrapods, the humerus is the principal upper limb support. In birds, it is lightweight and sometimes hollow to assist with flight mechanics. In aquatic mammals like whales, the humerus becomes remarkably short and stout, adapting to flipper locomotion, whereas running mammals like horses elongate and tuck the humerus closer to the body chest wall for efficient stride length.
Evolutionary History
Tracing back to early lobe-finned fish, the single upper limb bone emerged from ancestral fin supports. Over hundreds of millions of years of terrestrial evolution, this simple strut adapted to support body weight out of water, transitioning from crawling sprawls to upright, vertical limb postures.
Photography Tips
Lay the bone flat on a solid, neutral-background surface with a ruler or scale bar alongside it. Take one straight-down photo showing the entire length, and a second close-up shot focused specifically on the rounded top end and the bottom connecting surfaces.