Metacarpal or Metatarsal (Metapodial)
Long Bone; part of the limb (appendicular) skeleton
Also known as: Cannon Bone, Shin Bone, or Shank Bone
Classification
Long Bone; part of the limb (appendicular) skeleton
Dimensions
Generally long, slender, and straight with a cylinder-like middle section and a flattened, double-hinged bottom end. In medium to large grazers, these often range from 6 to 12 inches in length.
Key Features
The most striking feature is the bottom end, which looks like a pulley or a double-barrel hinge with a deep notch in the middle. The shaft is often slightly flattened on one side rather than perfectly round, and it may have a visible vertical groove running down its length reflecting its fused origins.
Similar Bones
This is easily confused with the humerus or femur. However, the cannon bone lacks the ball-and-socket joint found at the top of a thigh bone and follows a much straighter, more uniform path without the complex bumps and wide flaring seen in upper limb bones.
Across Species
In hoofed animals like deer and cattle, this structure is actually a fusion of two separate bones that would be in our hands or feet. This adaptation creates a long, sturdy lever that allows for efficient, high-speed running. While we have multiple separate bones in our palms and feet, these animals have evolved a single, heavily reinforced 'cannon bone' to support their weight on a single or double toe.
Evolutionary History
Deep in the past, the ancestors of modern grazers had five separate toes. Over millions of years, as these animals moved from soft forest floors to hard, open grasslands, the outer toes shrank away while the central bones grew longer and fused together. This 'stilted' limb design is one of nature's most successful solutions for escaping predators and traveling long distances across open plains.
Photography Tips
Place a ruler or common object like a coin next to the bone for size reference. Take one photo looking directly at the double-hinged bottom end and another of the entire length from the side to show the overall straightness of the shaft.