Calcaneus
Short bone, limb skeleton (hind limb)
Also known as: Heel bone, hock bone (in quadrupeds)
Classification
Short bone, limb skeleton (hind limb)
Dimensions
This bone is typically longer than it is wide, featuring a thick, blocky body and a prominent, elongated handle-like projection known as the heel process.
Key Features
It is easily recognized by its 'pistol-grip' or 'L-shaped' profile. One end has a smooth, scooped-out area that fits against other ankle bones, while the other end consists of a long, solid shaft with a slightly bulbous tip where large tendons attach.
Similar Bones
It can be confused with the Ulna (the outer forearm bone). However, the heel bone is much thicker relative to its length and lacks the deep, C-shaped notch found at the elbow end of an ulna. Compared to other ankle bones, it is the only one with a long, protruding tail-like section.
Across Species
In humans, this bone is short and wide to support upright walking by acting as a sturdy base. In hoofed mammals like deer or horses, it is much longer and more slender, forming the 'point' of the hock high up the leg. This extra length creates a powerful lever that allows for explosive jumping and running. In whales, the bone is greatly reduced as the hind limbs have mostly vanished.
Evolutionary History
The heel bone evolved from primitive ankle bones in early land-dwelling vertebrates. As mammals transitioned from flat-footed walking to walking on their toes (like cats or dogs) or on hooves, this bone stretched backward to provide better mechanical leverage for the muscles that push the foot off the ground.
Photography Tips
Place the bone on a solid, neutral background. Take one photo from the side to show the full 'L-shape' profile, and another looking straight down at the scooped joint surfaces. Placing a coin or ruler next to it helps viewers understand the size of the animal it came from.