G, The AChA has been elevated to expose a large perforating branch of the PComA called a premamillary artery. H, The PComA has been elevated to provide an excellent exposure of the basilar apex and the P1s. The ipsilateral SCA arises as a duplicate artery. I, The tentorium has been divided behind where the trochlear nerve enters the edge. This increases the length of basilar artery exposed. The trunks of a duplicate superior cerebellar artery loop down toward the trigeminal nerve. J, The petrous apex has been removed to complete an anterior petrosectomy approach, which increases access to the front of the brainstem and the basilar artery. In this case, the labyrinth including the cochlea and semicircular canals, and the nerves in the internal acoustic meatus have been exposed to show the relationship of the drilling for the anterior petrosectomy in relationship to these structures. The drilling for an anterior petrosectomy is directed behind the petrous carotid artery medial to the labyrinth and proceeds medially to the inferior petrosal sinus and side of the clivus. The abducens nerve and the ICA are in the lower margin of the exposure. (Images courtesy of AL Rhoton, Jr.)