Inferior View of the Carotid Canal and Skull Base. A, Inferior view of the carotid canal and skull base. The entrance into the carotid canal is located directly in front of the jugular foramen, posterolateral to the mandibular fossa and temporomandibular joint, and posterior to the sulcus tubae in which the eustachian tube sits. The canal extends forward and medial along the long axis of the petrous part of the temporal bone to the area above the foramen lacerum where it turns upward. B, A red tube has been placed in the carotid canal to show the course of the artery through the petrous part of the temporal bone and above the foramen lacerum. The interrupted lines show the position of the eustachian tube, which sits in the sulcus tubae below the junction of the sphenoid and temporal bones. The vidian nerve, represented by a yellow fiber, passes above the petrous apex and lateral to the foramen lacerum to reach the vidian canal and pterygopalatine fossa. C, Inferior view of an axial section of the skull base. The pharyngeal recess (fossa of Rosenmüller) projects laterally from the posterolateral corner of the nasopharynx below the foramen lacerum toward the carotid artery just below where the artery enters the carotid canal. The carotid artery enters the carotid canal on the lateral side of the longus capitis muscle and posterior to the fibrous and cartilaginous part of the eustachian tube and courses anteromedial toward the petrous apex and the area above the foramen lacerum where the anterior genu is located. The lower part of the pterygoid process has been removed to expose the maxillary artery in the pterygopalatine fossa. The pterygopalatine fossa is located between the posterior maxillary wall anteriorly and the pterygoid process posteriorly. The medial part of the eustachian tube has been removed. D, Enlarged view with highlighting of the pre- (red) and poststyloid (yellow) compartments of the parapharyngeal space. The styloid diaphragm, formed by the anterior part of the carotid sheath, separates the parapharyngeal space into pre- and poststyloid parts. The prestyloid compartment, a narrow fat-containing space between the medial pterygoid and tensor veli palatini muscle, separates the infratemporal fossa from the medially located lateral nasopharyngeal region containing the tensor and levator veli palatini muscles and the eustachian tube. The poststyloid compartment, located behind the prestyloid part, contains the internal carotid artery, internal jugular vein, and Cranial Nerves IX through XII. (Images courtesy of AL Rhoton, Jr.)