Foramen lacerum, left side. The clival, sphenoid, and pterygoid bones have been drilled to expose the contents of foramen. The foramen lacerum has 2 compartments: upper or vascular, and lower or fibrous. The upper compartment is occupied by the lacerum segment of the ICA, merely a transitional segment between petrosal and paraclival segments. The lower compartment is formed by the fibrous tissue of the pterygosphenoidal fissure, which attaches to the lacerum ICA superiorly and continues inferiorly with the pharyngobasilar fascia and the eustachian tube. The lingual process ofthe sphenoid bone, which has been removed, indents the lateral aspect of the upper compartment, while the vidian nerve enters the vidian canal at the lateral aspect of the lower compartment, but posteriorly it runs lateral to the petrous ICA. (Image courtesy of JC Fernandez-Miranda)