Humerus
Long Bone; Appendicular Skeleton (Limb)
Also known as: Upper Arm Bone
Classification
Long Bone; Appendicular Skeleton (Limb)
Dimensions
In mammals, the humerus is typically characterized by a long, tubular shaft with rounded, widened ends. Its length is usually around 1.2 to 1.5 times the length of the shoulder blade, and it serves as the primary support for the upper forelimb.
Key Features
Look for a smooth, rounded 'head' at the top that fits into the shoulder, which looks like half a marble. Along the upper shaft, there is often a rough ridge where muscles once attached. The bottom end is characterized by two distinct rounded bumps that act like a spool for a hinge-style elbow joint.
Similar Bones
The humerus is most often confused with the femur (thigh bone). To tell them apart, look at the top: the femur has a very distinct, long 'neck' extending to its ball-shaped head, whereas the humerus's head sits more directly on top of the shaft. Additionally, the elbow end of a humerus usually has more complex ridges compared to the smoother knee end of a femur.
Across Species
The humerus displays remarkable adaptations across nature. In digging animals like moles, it is extremely short, wide, and powerful. In birds, it is often hollow and light to aid in flight. In sea turtles and whales, it is flattened and shortened into a sturdy paddle-like structure, while in fast-running mammals like deer, it is long and slender to provide a greater stride length.
Evolutionary History
This bone can be traced back over 350 million years to the first lobe-finned fish that transitioned toward land. In these early pioneers, the humerus was a thick, stout plate. Over millions of years, as ancestors became more mobile on land, the bone elongated and rotated to lift the body off the ground, eventually evolving into the highly mobile ball-and-socket structure seen in modern mammals.
Photography Tips
To help with identification, place the bone on a plain, neutral background alongside a ruler or a common object like a coin for scale. Capturing a clear photo of the top rounded head and a separate one of the bottom elbow joint from the front and back will provide the most helpful clues.