External Craniocervical Ligaments

The anterior atlanto-occipital membrane is a wide, dense, fibroelastic band extending between the anterior margin of the foramen magnum and the upper border of the anterior arch of the atlas. Laterally, it is continuous with the articular capsules of the atlanto-occipital joints. In the midline, it is reinforced by the upward continuation of the anterior longitudinal ligament.

The posterior atlanto-occipital membrane is broader and thinner than the anterior one and connects the posterior margin of the foramen magnum with the upper border of the posterior arch of the atlas. On each side, it arches over the groove for the vertebral artery, leaving an opening for the upward passage of the artery and the outward passage of the first cervical spinal nerve.

Articular capsules surround the joints between the occipital condyles and the superior atlantal facets. The capsules are rather loose, allowing nodding movements of the head, and are thin medially; laterally, they are thickened and form the lateral atlanto-occipital ligaments, which limit lateral tilting of the head.

The anterior longitudinal ligament extends from the base of the skull to the sacrum. Its uppermost part reinforces the anterior atlanto-occipital membrane in the midline. The part between the anterior tubercle of the atlas and the anterior median ridge on the axis may have lateral extensions—the atlantoaxial (epistrophic) ligaments.

The ligamentum nuchae is a dense fibroelastic membrane stretching from the external occipital protuberance and crest to the posterior tubercle of the atlas and the spinous processes of all the other cervical vertebrae. It provides areas for muscular attachments and forms a midline septum between the posterior cervical muscles. The ligamentum nuchae is better developed in quadrupeds than in humans.

The ligamenta flava contain a high proportion of yellow elastic fibers and connect the laminae of adjacent vertebrae. They are present between the posterior arch of the atlas and the laminae of the axis but absent between the atlas and skull.

Intervertebral disks are lacking between the occiput and atlas and between the atlas and axis.

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