Manubrium segment of the Sternum
Flat Bone and part of the axial (core) skeleton
Also known as: Handle of the Breastbone
Classification
Flat Bone and part of the axial (core) skeleton
Dimensions
In humans, this top section is typically the widest and thickest part of the breastbone, being roughly shield-shaped or like a broad knot in a necktie.
Key Features
The highlighted top section (manubrium) is characterized by its wide, flared top edge which features a central dip or notch. It has distinct scalloped indentations on the outer sides where the collarbones and the first ribs join the structure.
Similar Bones
From a distance, it could be confused with a sacrum (the base of the spine), but the breastbone is much flatter and lacks the large holes and spinal nerve passages found in the sacrum. The central body segment (gladiolus) is also narrower and more rectangular than this top flared section.
Across Species
The breastbone varies greatly across the animal kingdom. In birds, it is shaped like a deep boat hull (the keel) to provide an anchor for massive flight muscles. In turtles, it forms the flat bottom plate of the shell (the plastron). In many quadrupeds, the breastbone is long and narrow, whereas in humans, it has become a broad, flat shield to accommodate the flatter shape of the chest.
Evolutionary History
This bone appeared as a way to unify the ribs and protect the internal organs as vertebrates moved onto land. In early four-legged ancestors, it was a cartilaginous structure, eventually hardening into the series of bony segments we see in mammals today, providing a stable foundation for the front limbs to attach via the collarbones.
Photography Tips
To identify this bone, take a clear photo of the front-facing flat surface. Use a top-down angle to show the notch at the top and the thickness of the bone. Placing a coin or ruler next to it helps determine the size compared to other mammal species.
Notes
What is the highlighted part of this bone?