Metacarpal
Long Bone; part of the limb (appendicular) skeleton
Also known as: Palm bone or Mid-hand bone
Classification
Long Bone; part of the limb (appendicular) skeleton
Dimensions
These are slender, tube-like bones that are generally longer than they are wide. In many mammals, they are roughly three to four inches long, featuring a flared base and a rounded top that connects to the fingers.
Key Features
A typical palm bone looks like a mini-pillar. It has a boxy base at the bottom, a straight shaft in the middle, and a smooth, rounded knob at the top that acts like a hinge for the knuckles. When looking at the back of the hand, these bones form the long ridges that lead from the wrist up to the base of each finger.
Similar Bones
They are often confused with metatarsals (the bones of the mid-foot). To tell them apart, remember that palm bones are generally flatter and more varied in length, whereas foot bones are more uniform, have a more arched shape, and are typically thicker to support the body's full weight.
Across Species
The mid-hand bones show incredible variety across the animal kingdom. In a horse, the third mid-hand bone has evolved into a massive, singular 'cannon bone' to support high-speed running. In bats, these bones are extremely elongated and thin, acting like the struts of an umbrella to support the wing membrane. Whales have shortened, flattened versions that form the internal structure of their stiff, paddle-like flippers.
Evolutionary History
These bones originated from the stout skeletal elements in the lobe-finned fish that first crawled onto land. Over millions of years, they transitioned from being weight-bearing structures for four-legged walking to specialized tools for grasping in primates. The thumb's mid-hand bone eventually gained a unique range of motion, allowing for the 'opposable' grip that defines many modern tree-dwelling and tool-using species.
Photography Tips
To capture the structure of these bones, photograph the hand resting flat against a neutral surface with a coin or ruler nearby for scale. Using side-lighting (light coming from the left or right) will cast shadows that highlight the long ridges of the shafts and the rounded knobs of the knuckles.