Calcaneus
Short bone; part of the limb (appendicular) skeleton
Also known as: Heel Bone or Hock Bone
Classification
Short bone; part of the limb (appendicular) skeleton
Dimensions
In many mammals, this bone is relatively long and blocky, acting as a sturdy lever. Its size varies greatly depending on the species, from the tiny nugget found in a rodent to the massive, heavy block in a large ungulate.
Key Features
This bone is easily identified by its chunky, irregular appearance and a prominent, extended handle-like projection known as the tuber. It often has smooth, scoop-like surfaces on one side where it fits against other ankle bones, while the rest of the surface is rough and textured for muscle attachment.
Similar Bones
It can be confused with the Talus (Astragalus), which sits right next to it in the ankle. The Talus is usually smoother and more pulley-shaped to allow for joint rotation, whereas the Calcaneus always has that distinctive, rugged 'heel' extension that acts as a lever.
Across Species
The shape of this bone is a masterclass in adaptation. In leaping animals like rabbits or deer, it is long and slender to provide maximum leverage for powerful jumps. In plantigrade animals, like bears or humans, it is wide and flat to provide a stable base for the foot to rest on the ground. In hoofed mammals like horses or cows, it is positioned higher up the leg, forming the 'point' of the hock, which is essentially the ankle being carried off the ground.
Evolutionary History
This bone originated as part of a cluster of small ancestral tarsal bones in early four-legged land dwellers. Over millions of years, as animals transitioned from sprawling to upright gaits, one particular bone lengthened to provide an attachment point for large leg muscles. This allowed for more efficient terrestrial movement and, eventually, the varied locomotion styles we see today, from sprinting to climbing.
Photography Tips
To identify this bone, take photos from multiple angles, especially a side view that shows the length of the heel projection and a view of the smooth joint surfaces. Always place a common object like a coin or a ruler next to the bone to provide a clear sense of its size.