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Brainstem and Right Posterior Fossa and Middle Fossa Structures

Surgical Correlation


Brainstem and right posterior fossa and middle fossa structures. The cerebrum has been dissected away to expose the brainstem and cerebellum 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 and petrous temporal bone 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. The tentorium cerebelli has been largely removed, except where it attaches to the petrous ridge and transverse groove on the occipital bone. Its tentorial notch or incisura is a U-shaped space that curves around the junction of the midbrain and pons to accommodate passage of the brainstem into the posterior fossa. The hyper-pigmented substantia nigra lies within the tegmentum of the midbrain posterior to the cerebral peduncles. Toward its dorsal surface in the midline is the narrow cerebral aqueduct that connects the third ventricle with the fourth ventricle. Ventral to the pons is the midline basilar artery with its superior cerebellar and terminal posterior cerebral branches. Medial to the anterior process of the left lesser wing of sphenoid is the optic nerve (CNII) emerging from the optic canal and the ipsilateral internal carotid artery. The oculomotor (CNIII) nerves pass through the interpeduncular fossa, and penetrate the dura between the anterior and posterior petroclinoid ligaments lateral to and in front of the posterior clinoid processes. These ligaments attach to their respective clinoid processes and the petrous apex. In their course the nerves pass caudal to the posterior cerebral artery, between this and the superior cerebellar artery. The right trochlear (CNIV) nerve is shown 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. The oculomotor and trochlear 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. Posterolateral to the trigeminal prominence, the roof of the internal auditory canal and the arcuate eminence have been drilled to expose the facial nerve as well as the bony labyrinth. The internal auditory meatus can be seen on the posteromedial surface of the petrous temporal bone, along with the facial and vestibulocochlear nerves entering it. The distal end of the auditory canal separates the cochlea, anteriorly, from the bony labyrinth and semicircular canals, posteriorly. In this view, the geniculate ganglion is exposed 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 trigeminal ganglion 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 been drilled away to reveal the middle ear cavity. The malleus and incus are exposed as well as the course of the chorda tympani nerve (a branch of the facial nerve) between the handle of the malleus and long process of the incus. 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. Laterally, the roof of the external auditory meatus has been drilled to demonstrate this canal. The tympanic membrane is the boundary between the external and middle ears. The bone covering the roof of the infratemporal fossa has been drilled to reveal the superior head of the lateral pterygoid muscle and branches of the mandibular nerve. The mandibular nerve divides into a smaller anterior trunk or division (not labeled) that provides motor branches to the muscles of mastication and the sensory buccal nerve to the cheek. A larger posterior trunk or division gives rise to motor fibers to the mylohyoid and anterior digastric muscles and sensory fibers to the auriculotemporal, lingual, and inferior alveolar nerves. 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)