Axis (anatomy)
second cervical vertebra of the spine
In anatomy, the axis (from Latin axis, "axle") is the second cervical vertebra (C2) of the spine, immediately inferior to the atlas, upon which the head rests. The spinal cord passes through the axis.
The defining feature of the axis is its strong bony protrusion known as the dens, which rises from the superior aspect of the bone.
Structure
The body is deeper in front or in the back and is prolonged downward anteriorly to overlap the upper and front part of the third vertebra.
It presents a median longitudinal ridge in front, separating two lateral depressions for the attachment of the longus colli muscles.
Dens
The dens, also called the odontoid process, or the peg, is the most pronounced projecting feature of the axis. The dens exhibits a slight constriction where it joins the main body of the vertebra. The condition where the dens is separated from the body of the axis is called os odontoideum and may cause nerve and circulation compression syndrome. On its anterior surface is an oval or nearly circular facet for articulation with that on the anterior arch of the atlas. On the back of the neck, and frequently extending on to its lateral surfaces, is a shallow groove for the transverse atlantal ligament which retains the process in position.
“Axis (anatomy)” enters the record as second cervical vertebra of the spine. Crown Archives preserves that source wording while asking what Axis, anatomy and second can confirm, complicate or overturn.
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This entry incorporates text from “Axis (anatomy)” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.