A, The craniotomy flap and dural opening exposes the temporal lobe and the floor of the middle cranial fossa. The insert shows the site of the scalp incision. B, The temporal lobe has been elevated to expose the PCA and SCA in the anterior and middle incisural space. The PCA passes above and the SCA below the oculomotor nerve. The SCA branches course with the trochlear nerve around the side of the brainstem. C, The PCA has been depressed to expose the basilar artery. The anterior choroidal artery arises in the anterior incisural space and passes between the cerebral peduncle and uncus to enter the crural cistern in the middle incisural space. D, The tentorium has been divided behind the petrous ridge to expose the SCA and the trigeminal and trochlear nerves in the region of the middle incisural space. The SCA sends branches above the trigeminal nerve and into the anterior part of the cerebellomesencephalic fissure. The medial posterior choroidal artery also passes around the lateral side of the brainstem. (Images courtesy of AL Rhoton, Jr.)