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Superior View of Right Middle and Inner Ear Structures, Lateral Cavernous Sinus, and Middle Fossa Floor

Surgical Correlation

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Superior view of right middle and inner ear structures, lateral cavernous sinus, and middle fossa floor. The cerebrum has been dissected away to expose the brainstem (top border of image) and cerebellum (left border of image) in situ. The dura mater has been stripped from the lateral surface of the sphenoid body and greater wing and a portion of the greater wing has been drilled. In this process, the lateral wall of the cavernous sinus has been exposed containing the oculomotor and trochlear nerves in its superior portion and the cavernous segment of the internal carotid artery. The oculomotor (CNIII) nerve is shown leaving the ventral midbrain, passing through the interpeduncular fossa, and penetrating the dura between the anterior and posterior petroclinoid ligaments lateral to and in front of the posterior clinoid process. In its course it passes caudal to the posterior cerebral artery, between this and the superior cerebellar artery, both branches of the midline basilar artery. The right trochlear (CNIV) nerve is seen leaving the dorsal surface of the midbrain and curving around the cerebral peduncle between the posterior cerebral and superior cerebellar arteries to gain the lateral wall of the cavernous sinus. Both nerves enter the orbit via the superior orbital fissure. The trigeminal nerve leaves the lateral surface of the pons and crosses the apex of the petrous bone where it then expands as the trigeminal ganglion. The ganglion lies in Meckel's cave on the trigeminal depression anteromedial to the trigeminal prominence, an elevation on the petrous ridge.  From this sensory ganglion arise its three divisions: ophthalmic (CNV1), maxillary (CNV2), and mandibular (CNV3) coursing toward their openings of exit; superior orbital fissure, foramen rotundum, and foramen ovale, respectively. Posterolateral to the foramen ovale is the foramen spinosum. The middle meningeal artery can be seen emerging from it. The bone covering the roof of the infratemporal fossa has been drilled to reveal the superior head of the lateral pterygoid muscle. Emerging from the foramen ovale is the smaller anterior division of the mandibular nerve and its larger posterior division. Posterolateral to the trigeminal prominence, the roof of the internal auditory canal has been drilled as well as the bony labyrinth. The distal end of the auditory canal separates the cochlea, anteriorly, from the bony labyrinth and semicircular canals, posteriorly. In this view, the facial and vestibulocochlear nerves are exposed and the geniculate ganglion is at the distal end of the canal. Here, the facial nerve continues posteriorly in the facial canal along the medial wall of the tympanic cavity inferior to the lateral semicircular canal and the greater superficial petrosal nerve takes its origin anteriorly from the ganglion. The greater superficial petrosal nerve leaves the anterior surface of the petrous bone through a hiatus and passes deep to the mandibular nerve toward the foramen lacerum and pterygoid canal to unite with the deep petrosal nerve to form Vidian nerve. The thin roof of the tympanic cavity (tegmen tympani) has also been drilled away to reveal the middle ear cavity (malleus and incus are exposed). The anterior wall of the tympanic cavity has been opened to reveal two canals, one above the other. The superior of the two contains the tensor tympani muscle and the lower one is the bony part of the Eustachian tube. The tensor tendon curves sharply laterally to attach to the upper part of the handle of the malleus. The tentorium has been incised to expose part of the rostral cerebellum. This surface is supplied by the medial and lateral branches of the superior cerebellar arteries that course posteriorly in the pontomesencephalic sulcus above the superior cerebellar peduncles before ramifying. The superior petrosal sinus is exposed in the tentorial attachment to the petrous ridge. It drains the cavernous sinus to the transverse sinus. (Image courtesy of PA Rubino)

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