Humerus
Long Bone; Part of the appendicular (limb) skeleton.
Also known as: Upper Arm Bone, Funny Bone, Wing Bone (in birds)
Classification
Long Bone; Part of the appendicular (limb) skeleton.
Dimensions
In mammals, this is a sturdy and elongated bone. It is typically one of the most robust bones in the front limb, featuring a rounded ball at the top and a spool-like shape at the bottom end.
Key Features
Look for a distinct rounded 'ball' on the top end that fits into the shoulder. The long shaft often has a slight spiral twist or a roughened area in the middle where muscles once attached. The bottom end is characterized by a spool-shaped surface and a deep pit on the back that allows the lower arm bone to lock in place when the limb is straightened.
Similar Bones
It is often confused with the femur (thigh bone). To tell them apart, look at the top: the humerus has a rounded head that sits directly on the shaft, while the femur usually has a distinct 'neck' that offsets the ball from the main bone. Additionally, the bottom of the humerus looks like a single spool, whereas the femur ends in two large, rounded rockers.
Across Species
The humerus is a master of adaptation. In moles, it is short and incredibly wide, acting like a powerful shovel for digging. In birds, it is hollow to reduce weight and provides the anchor for flight muscles. In aquatic mammals like whales and seals, the humerus has become shortened and flattened to form the internal framework of a stiff flipper, trading movement at the elbow for better leverage in the water.
Evolutionary History
This bone can be traced back over 350 million years to the early lobe-finned fish. In these ancestors, the humerus began as a stout, flat support for the pectoral fins. As vertebrates moved onto land, it elongated and rotated, eventually allowing for the diverse range of motions seen today, from the high-speed gallop of a horse to the complex grasping of a primate.
Photography Tips
Capture the bone from at least three angles: a front view, a side view to show the curve of the shaft, and a close-up of the bottom end. Avoid harsh overhead sunlight which can hide the subtle pits and ridges; soft, angled light is best. Including a common object like a coin or a ruler will help viewers understand the scale of the specimen.