Foramen magnum

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Structures of the cranial fossae.

The foramen magnum (Latin for “big hole”) is the largest foramen of the skull. It is a large central, oval-shaped opening that lies in the deepest region of the posterior cranial fossa and is the most prominent feature in the floor of this aspect of the cranial base. Specifically, it is enclosed within the anteromedian aspect of the occipital bone.

Externally, the anterolateral margins of the foramen magnum bear the occipital condyles which articulate with the superior articular facets on the lateral masses of the atlas (vertebra C1), permitting nodding movements. A prominent ridge called the external occipital crest also projects from the posterior margin of the foramen magnum in the midline to reach the external occipital protuberance of the occipital bone.

The foramen magnum transmits the spinal cord superiorly into the cranial cavity, where it becomes continuous with the lower end of the medulla oblongata, the terminal part of the brainstem. Apart from the spinal cord, its associated meninges and cerebrospinal fluid, the foramen magnum also transmits the vertebral arteries, the anterior and posterior spinal arteries, dural veins and the spinal roots of the accessory nerve (CN XI). Additionally, the apical ligament of the dens and the tectorial membrane, which is a superior continuation of the posterior longitudinal ligament, run through the foramen magnum to attach to the internal aspect of the occipital bone.