Humerus
Long Bone; Appendicular Skeleton (Limb)
Also known as: Upper Arm Bone
Classification
Long Bone; Appendicular Skeleton (Limb)
Dimensions
In most mammals, this is a sturdy, cylindrical bone. It is generally longer than the double bones of the lower arm but shorter and thicker than the thigh bone. Its length usually represents about 15-20% of the total skeletal height.
Key Features
Look for a rounded, smooth ball-shaped head at the top end that fits into the shoulder. The long middle section, or shaft, often has a slight twist or spiral-like ridge. At the bottom end, look for a spool-shaped surface that resembles a pulley, paired with two distinct bumps on the sides that feel like 'knuckles.'
Similar Bones
The femur (thigh bone) is the most common look-alike. You can tell them apart because the femur has a much longer, thinner 'neck' supporting its ball-shaped head, and the bottom end of a femur looks like two large, rounded wheels rather than a single pulley-like shape.
Across Species
The humerus is a versatile tool across the animal kingdom. In birds, it is often hollow and light to assist in flight, while in digging animals like moles, it is extremely short, wide, and powerful. Marine mammals like whales have a flattened, paddle-like humerus that stays fixed within their flippers. In fast-running hoofed animals like horses, the humerus is relatively short compared to the lower leg bones, allowing for an efficient, pendulum-like stride.
Evolutionary History
This bone can be traced back over 350 million years to the first lobe-finned fish that began exploring the shallows. As vertebrates moved onto land, the humerus shifted from a horizontal fin-support to a vertical pillar designed to lift the chest off the ground. Over millions of years, it has rotated and elongated to facilitate different types of movement, from swinging through trees to swimming in the deep ocean.
Photography Tips
Lay the bone flat on a plain, non-reflective surface. Take one photo from directly above to show the full length. Capture a second photo looking straight down at the round ball at the top, and a third looking at the pulley-shaped bottom. Including a common object like a coin or a ruler helps show the size.