Metapodial (Metatarsal or Metacarpal)
Long bone, part of the limb (appendicular) skeleton
Also known as: Cannon bone, Shin bone, Lower leg bone
Classification
Long bone, part of the limb (appendicular) skeleton
Dimensions
In deer and cattle, these are long and slender, typically measuring between six to twelve inches in length depending on the animal size. The shaft is usually flat or rounded with a thick, heavy bottom end containing two distinct knuckles.
Key Features
The most striking feature is the 'split' or double-knuckle at the bottom end, which resembles a pulley system. There is often a deep vertical groove running down the front and back of the shaft where tendons once rested, giving it the appearance of two fused tubes.
Similar Bones
It can be confused with the upper leg bones (humerus or femur), but the metapodial is much straighter with a very characteristic double-hinged joint at the bottom. It also lacks the ball-and-socket head found on the thigh bone.
Across Species
In even-toed ungulates like deer, sheep, and cows, this bone represents the fusion of two separate foot bones into one strong pillar to support weight. In contrast, horses have a single massive cannon bone (the third digit), while humans have five separate, smaller bones in the palm of the hand or arch of the foot.
Evolutionary History
This bone is a classic example of adaptation for speed and efficiency. Over millions of years, as ancestors of modern hoofed mammals moved from forests to open grasslands, their outer toes reduced and their central foot bones elongated and fused, allowing them to run on their tiptoes to escape predators.
Photography Tips
Capture the 'pulley' end from the very bottom to show the double-knuckle shape. Take a side profile photograph to show how flat or rounded the shaft is, and always include a ruler or a common object like a coin for scale.