Human Spine-Anatomy of Second Cervical Vertebra or Axis

Second cervical vertebra is also knows as axis. It is identified by the presence of the dens (see below).It has following features.

Body

1. The superior surface of the body is fused with the dens, and is encroached upon on each side by the superior articular facets. The dens articulates anteriorly with the anterior arch of the atlas, and posteriorly with the transverse ligament of the atlas.
2. The inferior surface has a prominent anterior margin which projects downwards.
3. The anterior surface presents a median ridge on each side of which there are hollowed out impressions.

Dens

Dens (also called odontoid process)is a strong, tooth-like process projecting upwards from the body of axis. The dens is usually believed to represent the centrum (body) of the atlas or first cervical vertebra which has fused with the centrum of the axis.

Vertebral Arch

1. The pedicles are concealed (superiorly) by the superior articular process. The inferior surface presents a deep and wide inferior vertebral notch, placed in front of the inferior articular process. The superior vertebral notch is very shallow and is placed on the upper border of the lamina, behind the superior articular process.
2. The laminae are thick and strong.
3. Articular facets. Each superior articular facet occupies the upper surfaces of the body and of the massive pedicle. Laterally it overhangs the foramen transversarium. It is a large, flat, circular facet which is directed upwards and laterally. it articulates with the inferior facet of the atlas vertebra to form the atlanto-axial joint. Each inferior articular facet lies posterior to the transverse process and is directed downwards and forwards to articulate with the 3rd cervical vertebra.
4. The transverse processes are very small and represent the true posterior tubercles only. The foramen transversarium is directed upwards and laterally.
5. The spine is large, thick and very strong. It is deeply grooved inferiorly. Its tip is bifid, terminating in two rough tubercles.

Attachments

• The dens provides attachment at its apex to the apical ligament, and on each side (below the apex) to the alar ligaments.
• The anterior surface of the body receives the insertion of the longus colli. The anterior longitudinal ligament is also attached to the anterior surface.
• The posterior surface of the body provides attachment, from below upwards, to the posterior longitudinal ligament, the membrana tectoria and the vertical limb of the cruciate ligament.

• The laminae provide attachment to the ligamenta flava.
• The transverse process gives origin by its tip to the levator scapulae, the scalenus medius (anteriorly) and the splenius cervicis (posteriorly). The intertransverse muscles are attached to the upper and lower surfaces of the process.
• The spine gives attachment to the ligamentum nuchae; the semispinalis cervicis, the rectus capitis posterior major, the inferior oblique; the spinalis cervicis, the interspinalis and the multifidus.

Text adapted from: Human Anatomy by BD Chaurasia

Image Credit: Wikipedia

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