Phalanges and Metacarpals
Long bones within the limb (appendicular) skeleton
Also known as: Finger and Hand bones (sometimes called 'Knuckle bones' or 'Digit bones')
Classification
Long bones within the limb (appendicular) skeleton
Dimensions
In primates, these bones are generally slender and reach about 2 to 4 inches in length, forming a fan-like structure that is wider at the base and narrows toward the tips.
Key Features
Look for a series of cylindrical shafts with rounded, spool-shaped knobs at the joints. The bones at the palm area are longer and have flatter bases, while the finger segments become progressively smaller and shorter toward the tips. The very last bones in the sequence often have a flattened, spade-like tip where the nail or claw attaches.
Similar Bones
These can be confused with the bones of the feet (metatarsals and toe phalanges). However, hand bones are typically more slender and have a greater range of motion at the joints, whereas foot bones are thicker and blockier to support the weight of the body.
Across Species
The layout of these bones is a classic example of the pentadactyl limb, a plan shared by many vertebrates. In bats, these bones are extremely elongated to support wing membranes. In contrast, in horses, the middle digit has become thickened and central to form the cannon bone and hoof, while the side digits have diminished or vanished. Whales have these bones encased in flesh to form sturdy paddles for steering in water.
Evolutionary History
These bones trace back to the lobe-finned fish of the Devonian period. Over millions of years, the sprawling fins of early tetrapods evolved into specialized grasping or weight-bearing digits. The evolution of the 'opposable' first digit provided a significant advantage for climbing and tool use, allowing for precision grips that differ from the power grips seen in many other mammals.
Photography Tips
Lay the assembly flat on a neutral background. Take one photo directly from above to show the overall fan shape and spacing. Include a second photo from the side to show the natural curve of the bones and the shape of the knuckle joints. Placing a coin or ruler nearby helps show the relative size.