The foramen magnum is the largest foramen of the skull and is part of the occipital bone . It is oval in shape with a large anteroposterior diameter .
The foramen magnum is found in the most inferior part of the posterior cranial fossa . It is traversed by vital structures including the medulla oblongata . 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
- medulla oblongata
- spinal root of the accessory nerve
- vertebral arteries
- anterior and posterior spinal arteries
- tectorial membrane and alar ligaments
- at the level of the foramen magnum CSF is seen to surround the medulla which is reduced or absent in tonsillar herniation