Axial view of the head at level of the midbrain. The cerebrum and cerebellum have been dissected away and the brainstem preserved in situ in this specimen. The hyper-pigmented substantia nigra and red nuclei lie within the tegmentum of the midbrain posterior to the cerebral peduncles. The oculomotor (CNIII) nerves can be seen leaving the midbrain, passing through the interpeduncular fossa toward the lateral wall of the cavernous sinus. In their course they pass caudal to the posterior cerebral artery, between this and the superior cerebellar artery. The trochlear (CNIV) nerves leave the dorsal surface of the midbrain and curve around the cerebral peduncles to gain the lateral wall of the cavernous sinus. The sensory and motor roots of the trigeminal (CNV) nerve leave the lateral pons and cross the petrous apex to enter Meckel's cave. Here, the trigeminal (Gasserian) ganglion sits on the trigeminal impression. From the convex margins of the ganglion arise the three branches or divisions of this nerve: ophthalmic (CNV1), maxillary (CNV2), and mandibular (CNV3). At the lateral margin of the pontomedullary junction arise the facial (CNVII) and vestibulocochlear (CNVIII) nerves, which enter the internal auditory canal. Its roof has been drilled to expose the course of these nerves within. The labyrinthine segment of the facial nerve ends at the geniculate ganglion, from which originates the greater superficial petrosal nerve. From here, the tympanic segment of the facial nerve curves posteriorly to enter the facial canal along the medial wall of the tympanic cavity and passes inferior to the lateral semicircular canal before bending inferiorly as the mastoid segment (shown on the left side of the image). The optic (CNII) nerves emerge out of the optic canals and cross at the optic chiasm. The optic nerve and chiasm connect the retinal fibers to the optic tracts. The basilar artery terminates as the posterior cerebral arteries (precommunicating, P1 segment). These receive the posterior communicating arteries from the internal carotid arteries to complete the posterior portion of the circle of Willis before curling around the cerebral peduncles (postcommunicating, P2 segment) above the tentorium. They ultimately supply the occipital lobes and posteromedial portions of the temporal lobes. The internal carotid arteries terminate by dividing into the anterior and middle cerebral arteries. The anterior cerebral arteries (precommunicating, P1 segments) course medially toward the longitudinal fissure. Here, the right and left arteries are joined by a short anterior communicating artery to complete the anterior portion of the circle of Willis. The postcommunicating, P2 segments of the arteries continue forward along the medial surfaces of the frontal lobes. (Image courtesy of AL Rhoton, Jr.)