Phalanx
Long bone of the appendicular skeleton
Also known as: Toe bone, Finger bone, Knuckle bone
Classification
Long bone of the appendicular skeleton
Dimensions
These are usually small, tube-like bones that are shorter than the rods of the palm or foot. They feature a flared base and a rounded, spool-shaped head at the opposite end.
Key Features
Look for a distinct 'hour-glass' or 'dog-bone' shape. One end (the top) usually has a shallow bowl or cup-like surface to receive a rounded joint, while the other end (the bottom) typically looks like a small pulley or a pair of rounded knuckles.
Similar Bones
They are easily confused with metacarpals or metatarsals (palm and sole bones). You can tell a phalanx apart because it is much shorter in proportion to its width and the pulley-shaped end is much more pronounced for the wide range of movement needed in toes and fingers.
Across Species
The variety of these bones across the animal kingdom is staggering. In horses, the middle and end phalanges are encased in a single large hoof, effectively walking on their middle fingernail. In bats, these bones are incredibly elongated and thin to support the wing membrane, acting like the ribs of an umbrella. Whale flippers contain many extra phalanges to provide a wide, paddle-like surface for swimming.
Evolutionary History
Phalanges emerged as lobe-finned fish transitioned to life on land, allowing them to grip and push off against solid surfaces. Over millions of years, the number of these bones stabilized at five digits for many land dwellers, though many lineages have since lost or merged digits to specialize for running, flying, or swimming.
Photography Tips
Lay the bone flat against a neutral background. Take one photo from directly above to show the overall silhouette, and another from the side to capture the curve of the shaft and the shape of the joint ends. Always include a coin or a ruler for scale.