Humerus
A Long Bone belonging to the appendicular skeleton (the limbs).
Also known as: Upper Arm Bone, Wing Bone (in birds), Funny Bone (referring to the lower end)
Classification
A Long Bone belonging to the appendicular skeleton (the limbs).
Dimensions
In many mammals, this is a sturdy, cylindrical shaft with expanded endings. In humans, it is usually roughly a foot long, while in small rodents it may be less than an inch. It is typically the largest and strongest bone of the upper limb.
Key Features
The top end features a smooth, rounded ball shape that fits into the shoulder socket. The shaft is relatively straight but usually has a slight twist or ridge for muscle attachment. The bottom end—where the teal dot is located—is very distinctive, featuring two smooth rollers that resemble a spool of thread or a set of pulleys for the elbow joint.
Similar Bones
The femur (thigh bone) is the most common confusion. You can tell the humerus apart because its lower end looks like a pulley or a spool, whereas the femur's lower end has two large, rounded knobs like rocking chair runners. Additionally, the humerus is usually thinner and lighter than the femur.
Across Species
The humerus shows incredible diversity across the animal kingdom. In digging animals like moles, it is extremely short, broad, and robust to provide leverage for soil displacement. In birds, the bone is often pneumatic, meaning it is hollow and filled with air sacs to aid in flight. Whales have a very short, flattened humerus that serves as the internal structural support for their front flippers.
Evolutionary History
This bone can be traced back over 350 million years to the first tetrapods that crawled onto land. It evolved from the primary fin bones of lobe-finned fish. Over eons, it transitioned from a flattened paddle support into a weight-bearing pillar that allowed animals to lift their bodies off the ground and eventually develop the complex rotational movements seen in primates.
Photography Tips
To get the best identification, take photos from three angles: one of the round ball at the top, one of the 'pulley' shape at the bottom, and a side view showing the full length. Include a common object like a coin or a ruler next to the bone to provide a clear sense of scale.
Notes
What is the highlighted part?