Humerus
Long Bone (limb skeleton)
Also known as: Upper arm bone, Wing bone
Classification
Long Bone (limb skeleton)
Dimensions
Usually the largest and thickest bone of the upper limb; it is characterized by a long, cylindrical shaft with rounded, widened ends for joint movement.
Key Features
Look for a very smooth, rounded globe-like 'head' at the top end and a wider, flatter bottom end with spool-like bumps that resemble a pulley. The shaft is generally straight but may have a slight twist or a prominent ridge in some species.
Similar Bones
It is often confused with the femur (thigh bone). To tell them apart, check the top: the humerus usually has a more integrated, less distinct neck than the femur, and the bottom end of the humerus has a thin, spool-like shape rather than two large, rounded rollers.
Across Species
In birds, this bone is often hollow and light to allow for flight, while in turtles and tortoises, it is very thick and strongly curved to support the weight of a heavy shell. In burrowing mammals like moles, it is extremely short and wide, resembling a broad paddle to provide leverage for digging into the earth.
Evolutionary History
This bone can be traced back over 350 million years to the early lobe-finned fish. As these ancestors moved onto land, the single large bone at the top of the fin evolved into the sturdy anchor for the entire front limb, allowing for the diverse types of locomotion seen in land animals today.
Photography Tips
Lay the bone on a neutral background next to a familiar object like a coin or ruler for size. Take photos from the front, back, and both ends, making sure the rounded head at the top is clearly in focus.