||It has been suggested that Dens (anatomy) be merged into this article. (Discuss) Proposed since January 2014.|
Position of axis (shown in red).
Second cervical vertebra, or epistropheus, from above.
|Latin||Axis, vertebra cervicalis II|
|Anatomical terms of bone|
The most distinctive characteristic of this bone is the strong odontoid process ("dens") which rises perpendicularly from the upper surface of the body. That peculiar feature gives to the vertebra a rarely used third name: vertebra dentata. In some judicial hangings the odontoid process may break and hit the medulla oblongata, causing death.
The body is deeper in front than behind, and prolonged downward anteriorly so as to overlap the upper and front part of the third vertebra.
Its under surface is concave from before backward and convex from side to side.
The pedicles are broad and strong, especially in front, where they coalesce with the sides of the body and the root of the odontoid process. They are covered above by the superior articular surfaces.
The superior articular surfaces are round, slightly convex, directed upward and laterally, and are supported on the body, pedicles, and transverse processes.
The axis is ossified from five primary and two secondary centers.
The centers for the arch appear about the seventh or eighth week of fetal life, while the centers for the body appear in about the fourth or fifth month.
The dens or odontoid process consists originally of a continuation upward of the cartilaginous mass, in which the lower part of the body is formed.
About the sixth month of fetal life, two centers make their appearance in the base of this process: they are placed laterally, and join before birth to form a conical bilobed mass deeply cleft above; the interval between the sides of the cleft and the summit of the process is formed by a wedge-shaped piece of cartilage.
The base of the process is separated from the body by a cartilaginous disk, which gradually becomes ossified at its circumference, but remains cartilaginous in its center until advanced age.
In this cartilage, rudiments of the lower epiphysial lamella of the atlas and the upper epiphysial lamella of the axis may sometimes be found.
The apex of the odontoid process has a separate center which appears in the second and joins about the twelfth year; this is the upper epiphysial lamella of the atlas.
In addition to these there is a secondary center for a thin epiphysial plate on the under surface of the body of the bone.
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This article uses anatomical terminology; for an overview, see anatomical terminology.
- Netter, Frank. Atlas of Human Anatomy, "High Cervical Spine: C1-C2"