Cervical Vertebra (Axis or C2)
Irregular Bone; part of the axial (core) skeleton.
Also known as: Neck Bone, Second Vertebra, The Axis
Classification
Irregular Bone; part of the axial (core) skeleton.
Dimensions
Generally small compared to bones of the lower back, it is characterized by a central ring-like structure with several protruding wings and a distinct upward-pointing peg.
Key Features
The arrow points to the spinous process, which is the bony tail extending from the back of the vertebra. In the neck region, this 'tail' is often split or forked like a snake's tongue. The bone also features two side wings with small holes for blood vessels and a unique upward-pointing nub on the front section.
Similar Bones
It can be confused with other neck vertebrae, but it is the only one with a large upward-pointing tooth-like projection. It is much smaller and more delicate than the chunky, bean-shaped vertebrae found in the lower back.
Across Species
In many mammals, this bone is specialized to allow for head rotation. In long-necked animals like giraffes, these bones are greatly elongated, though they still have only seven neck vertebrae just like humans. In birds, the equivalent bones are far more numerous, providing the extreme flexibility seen in owls and herons.
Evolutionary History
This bone represents a major innovation in terrestrial vertebrates. Early tetrapods had limited neck mobility; the development of a specialized second vertebra allowed land-dwellers to move their heads independently of their shoulders, which was critical for hunting and spotting predators.
Photography Tips
To identify this bone effectively, take one photo looking down through the central hole to show the 'forked tail' and side holes, and another from the side to show the upward-pointing peg.
Notes
What is the arrow pointing to?