Foramen magnum

Last revised by Derek Smith on 11 Sep 2021

The foramen magnum is the largest foramen of the skull and is part of the occipital bone 1. It is oval in shape with a large anteroposterior diameter 2.

The foramen magnum is found in the most inferior part of the posterior cranial fossa 3. It is traversed by vital structures including the medulla oblongata 1. In the midline, the anterior margin is the basion and the posterior margin is the opisthion.

  • anteriorly: basilar part of occipital bone
  • anterolaterally: occipital condyles, hypoglossal canal, jugular foramen
  • posteriorly: squamous part of occipital bone with the internal occipital crest
  • at the level of the foramen magnum CSF is seen to surround the medulla which is reduced or absent in tonsillar herniation

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