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Brainstem, Fourth Ventricle, and Petrosal Cerebellar Surface: Stepwise Anterior Exposure

Surgical Correlation

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A, The petrosal surface faces forward toward the posterior surface of the temporal bone. The fourth ventricle is located behind the pons and medulla. The midbrain and pons are separated by the pontomesencephalic sulcus and the pons and medulla by the pontomedullary sulcus. The trigeminal nerves arise from the midpons. The abducens nerve arises in the medial part of the pontomedullary sulcus, rostral to the medullary pyramids. The facial and vestibulocochlear nerves arise at the lateral end of the pontomedullary sulcus immediately rostral to the foramen of Luschka. The hypoglossal nerves arise anterior to the olives and the glossopharyngeal, vagus, and accessory nerves arise posterior to the olives. Choroid plexus protrudes from the foramen of Luschka behind to the glossopharyngeal and vagus nerves. B, Right cerebellopontine angle following removal of some of the medulla. The foramen of Luschka opens into the cerebellopontine angle below the junction of the facial and vestibulocochlear nerves with the lateral end of the pontomedullary sulcus. Choroid plexus protrudes from the lateral recess and foramen of Luschka behind the glossopharyngeal, vagus, and accessory nerves. The cerebellopontine fissure, a V-shaped fissure formed by the cerebellum wrapping around the pons and middle cerebellar peduncle, has a superior and inferior limb that define the margins of the cerebellopontine angle. The superior limb extends above the trigeminal nerve and the inferior limb passes below the flocculus and the nerves that pass to the jugular foramen. C, The part of the pons and medulla forming the left half of the floor of the ventricle has been removed to expose the fastigium, which divides the ventricular roof into superior and inferior parts. D, The right half of the pons has been removed to expose the upper half of the roof. The superior part of the roof is formed by the superior medullary velum. The rostral part of the lower half of the roof is formed by the nodule and inferior medullary velum and the caudal part is formed by the tela choroidea, a thin arachnoid-like membrane, in which the choroid plexus arises. E, The cerebellopontine fissure has upper and lower limbs, which meet at a later apex located at the medial end of the petrosal fissure, also called the horizontal fissure, which divides the petrosal surface into upper and lower halves. The junction of the pons and medulla, which forms the anterior wall of the left lateral recess, has been removed to expose the choroid plexus protruding through the lateral recess into the cerebellopontine angles. F, Enlarged view. The choroid plexus protrudes laterally through the foramen of Luschka into the cerebellopontine angle below the flocculus. (Images courtesy of AL Rhoton, Jr.)

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