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Accessory transverse foramen of the atlas is present in 1.4–12.5% across the population. [7] Foramen arcuale or a bony bridge above the vertebral artery on the posterior arch of the atlas may be present. This foramen has an overall prevalence of 9.1%. [8]
The transverse ligament divides the vertebral foramen of the axis into an anterior portion (constituting one third of its lumen) which contains the dens, and a posterior portion (constituting two thirds of the foramen's lumen) which contains the spinal cord and its coverings [1] as well as the two accessory nerves (CN XI). [2]
They then proceed superiorly, in the transverse foramen of each cervical vertebra. [1] Once they have passed through the transverse foramen of C1 (also known as the atlas), the vertebral arteries travel across the posterior arch of C1 and through the suboccipital triangle [3] before entering the foramen magnum. [1]
The atlas (C1) is the topmost vertebra, and along with the axis forms the joint connecting the skull and spine. It lacks a vertebral body, spinous process, and discs either superior or inferior to it. It is ring-like and consists of an anterior arch, posterior arch, and two lateral masses. The axis (C2) forms the pivot on which the atlas rotates.
Specific to the cervical vertebra is the transverse foramen (also known as foramen transversarium). This is an opening on each of the transverse processes which gives passage to the vertebral artery and vein and a sympathetic nerve plexus. On the cervical vertebrae other than the atlas, the anterior and posterior tubercles are on either side of ...
The vertebral foramen is large, but smaller than the atlas. The transverse processes are very small, and each ends in a single tubercle. Each process is perforated by the transverse foramen , which is directed obliquely upward and laterally.
The cruciate ligament of the atlas consists of the transverse ligament of the atlas, a superior longitudinal band, and an inferior longitudinal band. [1] [2] The superior longitudinal band connects the transverse ligament to the anterior side of the foramen magnum (near the basilar part) in the occipital bone of the skull.
The membrane situated is posterior/deep to the transverse ligament of the atlas; the two are separated by a thin intervening layer of loose areolar connective tissue, and sometimes by a bursa. [2] The membrane is situated anterior/superficially to the spinal dura mater (which is firmly attached to the tectorial membrane). [3]