Cervical vertebrae

In vertebrates, cervical vertebrae (singular: vertebra) are the vertebrae immediately below the skull.

Thoracic vertebrae in all mammalian species are those vertebrae that also carry a pair of ribs, and lie caudal to the cervical vertebrae. Further caudally follow the lumbar vertebrae, which also belong to the trunk, but do not carry ribs. In reptiles, all trunk vertebrae carry ribs and are called dorsal vertebrae.

In many species, though not in mammals, the cervical vertebrae bear ribs. In many other groups, such as lizards and saurischian dinosaurs, the cervical ribs are large; in birds, they are small and completely fused to the vertebrae. The transverse processes of mammals are homologous to the cervical ribs of other amniotes.

In humans, cervical vertebrae are the smallest of the true vertebrae, and can be readily distinguished from those of the thoracic or lumbar regions by the presence of a foramen (hole) in each transverse process, through which the vertebral artery passes.

Axis (anatomy)

In anatomy, the second cervical vertebra (C2) of the spine is named the axis (from Latin axis, "axle") or epistropheus.

It forms the pivot upon which the first cervical vertebra (the atlas), which carries the head, rotates.

The most distinctive characteristic of this bone is the strong odontoid process known as the 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.

Structure

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.

It presents in front a median longitudinal ridge, separating two lateral depressions for the attachment of the Longus colli muscles.

Its under surface is concave from before backward and convex from side to side.

Dens

The dens, also odontoid process or peg, is the most pronounced feature, and exhibits a slight constriction or neck where it joins the main body of the vertebra. The dens is a protuberance (process or projection) of the axis (second cervical 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. The apex is pointed, and gives attachment to the apical odontoid ligament; below the apex the process is somewhat enlarged, and presents on either side a rough impression for the attachment of the alar ligament; these ligaments connect the process to the occipital bone.

Atlas (anatomy)

In anatomy, the atlas (C1) is the most superior (first) cervical vertebra of the spine.

It is named for the Atlas of Greek mythology, because it supports the globe of the head .

The atlas is the topmost vertebra and with the axis forms the joint connecting the skull and spine. The atlas and axis are specialized to allow a greater range of motion than normal vertebrae. They are responsible for the nodding and rotation movements of the head.

The atlanto-occipital joint allows the head to nod up and down on the vertebral column. The dens acts as a pivot that allows the atlas and attached head to rotate on the axis, side to side.

The atlas's chief peculiarity is that it has no body. It is ring-like and consists of an anterior and a posterior arch and two lateral masses.

The atlas and axis are important neurologically because the brain stem extends down to the axis.

Structure

Anterior arch

The anterior arch forms about one-fifth of the ring: its anterior surface is convex, and presents at its center the anterior tubercle for the attachment of the Longus colli muscles and the anterior longitudinal ligament; posteriorly it is concave, and marked by a smooth, oval or circular facet (fovea dentis), for articulation with the odontoid process (dens) of the axis.