Sternum with Costal Cartilages
Flat bone of the core (axial) skeleton
Also known as: Breastbone and Rib Cartilage
Classification
Flat bone of the core (axial) skeleton
Dimensions
The central shield-like plate is typically a few inches long, with multiple semi-flexible finger-like extensions reaching outward to meet the ribs.
Key Features
The most striking feature is the central vertical plate with several pairs of curved, translucent-looking branches extending from the sides. The center often has a rounded top and a flared bottom, resembling a necktie or a dagger. The side branches are usually smoother and less dense than the central bone.
Similar Bones
It might be confused with a section of the spine (vertebrae), but the breastbone is generally flatter and does not have a hollow central hole for a spinal cord. Individual rib branches might look like small finger bones, but they lack the distinct joints and knuckles found in hands or feet.
Across Species
In birds, the sternum is often shaped like a massive keel to provide a surface for flight muscles. In turtles, it forms part of the flat bottom shell known as the plastron. Many mammals have a segmented breastbone that resembles a row of beads, while humans and some large mammals have a broad, flat plate designed to support the chest cavity.
Evolutionary History
The breastbone first appeared in early land-dwelling tetrapods to help support the body's weight against gravity. Over millions of years, it has transformed from a simple strip of cartilage into a sturdy bony anchor that protects the heart and lungs while allowing the chest to expand and contract during breathing.
Photography Tips
Lay the specimen against a flat, dark background to highlight the translucent edges. Take one photo directly from above to show the overall fan-like shape, and a second photo from the side to show how flat the central plate is. Including a coin or a ruler nearby helps viewers understand the scale.