Illustrated Encyclopedia of Human Anatomic Variation: Opus V: Skeletal Systems: Vertebral Column

Cervical Vertebrae

Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD
Peer Review Status: Internally Peer Reviewed

The cervical vertebrae exhibit considerable variation in their spinous processes. As a rule, in the White race, the second, third, fourth, and fifth vertebrae possess bifid spines.

The sixth and seventh demonstrate a tendency to be bifid with two small lateral tubercles. The sixth occasionally has a bifid spine, the seventh rarely. Occasionally all the cervical spines, with the exception of the second, are nonbifid.

In the "colored races", the cervical spines are relatively shorter than in whites generally and, as a rule , are simple. The only cervical vertebra that presents a bifid spine in all races is the axis. It may be nonbifid in the Black race, but rarely so in the white race.

Blocked or fused cervical vertebra may occur. In one study, three fusions were found between the second and third vertebra (3%) and two between the fifth and sixth vertebra (2%). The blocked vertebra exhibited fusion of vertebral bodies and laminae and the spines, transverse processes and pedicles remained independent. Associated vertebral anomalies include incomplete sacralization of the fifth lumbar and large sacral hiatuses. A hypoplastic intervertebral disc was visible between the fused bodies but there was no trace of a joint space between articular facets.

The laminae of the inferior cervical vertebrae frequently exhibit dorsally distinct tubercles from which fasciculi of the multifidus muscle arise. The tubercles may be found on the fourth and fifth vertebrae, but more frequently on the sixth and seventh. Anterior tubercles are bony projections that may be seen, usually, on the sixth cervical in lateral views of cervical spines.

A foramen transversarium, typical of the foramina of the cervical vertebral transverse processes, has been found in the first lumbar, fifth lumbar, and first sacral vertebrae.

An accessory transverse foramen, posterior to and smaller than the primary foramen, may be found in the sixth vertebra, less frequently in adjacent vertebrae

The transverse foramen of the seventh cervical is typically small and may be absent. It rarely transmits the vertebral artery but frequently allows passage of a vein.

The atlas of humans, compared with other animals, is considered a "degenerate bone."

The atlas may show incomplete ossification of the anterior and posterior arch. The absence of the anterior arch has been reported in an adult. The posterior arch may possess facets, which articulate with facets on the edge of the foramen magnum. The anterior arch may also have facets articulating with projections on the occipital bone.

One commonly finds the groove for the vertebral artery converted to a foramen. Paired ossicles have been reported in the posterior atlanto-occipital membrane, and they have been interpreted as vestiges of a proatlas.

Fusion, either partial or complete, of the atlas with the occipital bone may occur (incidence about 1%) (so-called atlas assimilation).

Regarding the axis, the dens may form a separate os odontoideum.

The sulcus arteriae vertebralis of the posterior arch may be bridged over by a bony process from the inferior articular process, producing a bony canal for the vertebral artery (10% of cases). A retrotransverse groove (or canal) may occasionally be present on the lateral mass of the atlas, posterior to the root of its transverse process. In one study (Gupta, et al., 1979) of 123 dry atlases the incidence was 44%. The groove was present in 31 (25%) specimens, canal in 14 (11.4%) specimens and canal and groove in 9 (7%) specimens. In another study (Veleanu, et al., 1977) of Romanian atlases the incidence was about 68%. It houses an anastomotic vein.

The posterior tubercle of the atlas is rarely bifid but may occasionally present two tubercles.

The atlas may be fused with the occipital bone in varying degrees. The "assimilation" of the atlas may occur in about 0.5 - 1% of skulls (1/157).

The dens of the axis may (a) develop as a separate bone, the os odontoideum; (b) arise from the anterior arch; (c) articulate with a third occipital condyle; or (d) be absent.

The axis (epistropheus) may be fused with either the atlas or with the third vertebra.

The vertebrarterial foramen associated with the sixth cervical vertebra occasionally appears as a notch and may also be associated with a secondary foramen for the vertebral vein.

The seventh cervical vertebra (vertebra prominens) is responsible for the palpable prominence ot the dorsum of the neck in 70% of individuals; the sixth cervical is the next most frequent in forming this prominence (20%), and the spinous process of the first thoracic is the least frequent (10%).

The ligamentum nuchae may contain sesamoid bone.

The transverse process of the seventh vertebra may be bifid, and occasionally the costal process is replaced by a cervical rib. The body of the seventh sometimes bears bilaterally, near its caudal border, a costal pit for the head of the first rib. When a costal pit is present there is usually a well-developed cervical rib. Cervical ribs associated with the seventh vertebra appear in a variety of forms. They may be fully formed or merely enlargements of costal processes. In very rare cases there is a a complete pair of ribs, which may articulate with the sternum. They may be rudimentary, appearing as a fibrous band that joins the usual first thoracic rib or first rib cartilage. The cervical rib may be unattached at its ventral (distal) end.

Rudimentary cervical ribs have been have been observed in one case on the sixth cervical vertebra, in two cases on the fourth, but never on the fifth. Cervical ribs are typically present in reptiles and birds.

Varieties of Different Vertebrae

Varieties of Atlas

Varieties of Axis

Atlas and Axis

Image 05

Unusual Absence of Anterior Arch of Atlas

Cervical Ribs

Cervical Ribs

Supernumerary Cervical Ribs

References

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