Trigeminal Ganglion and Regional Anatomy. A, Anterior view of the brain stem. The middle cerebellar peduncle is separated from the pons by a lateral pontine sulcus. Just lateral to the lateral pontine sulcus is the emergence of the trigeminal nerve. From the microsurgical standpoint, the apparent origin of the trigeminal nerve can be considered as the limit between the pons and the middle cerebellar peduncle. B, Lateral view of the right trigeminal root entry zone. The tentorium and the occipital lobe have been removed to expose the root entry zone of the trigeminal nerve. The small roots emerge from the pons superomedial to the large sensory root. The roof of the internal acoustic canal has been removed. C, The two motor rootlets groups of the trigeminal nerve, a primary superior and a secondary inferior motor rootlets, are visualized after superior retraction of the superior motor group from the sensory root. D, The dura of the anterior aspect of the posterior fossa has been removed to expose the basilar plexus, cavernous sinus, and inferior petrosal sinus. Meckel’s cave is a cleft-like dural pocket that originates from the dura propria of the posterior fossa. The cave is situated at the trigeminal impression between the meningeal layer (dura propria) and the periosteal layer of dura. E, The arachnoid membrane from the posterior fossa extends to Meckel’s cave, forming a pocket within the cave, continues along the rootlets of the trigeminal nerve. F, The arachnoid membrane over the trigeminal root has been removed. The superior petrosal sinus extends medially through the upper edge of the porus of Meckel’s cave and above the trigeminal nerve to join the cavernous sinus. (Images courtesy of AL Rhoton, Jr.)