Endoscopic view of left anterior pontomedullary region. Extensive bone removal has exposed the anterior brainstem and adjacent structures. The pons is visible with its overlying midline basilar artery formed by union of the vertebral arteries at the pontomedullary sulcus. Emerging from the ventral pons in this area is the abducens (CNVI) nerve. More superior and lateral on the pons (above the middle cerebellar peduncle) is the emergence of the trigeminal (CNV) nerve. This nerve courses over the petrous apex to Meckel's cave. The petrous bone has been removed to expose the internal carotid artery. Lateral to the emergence of the abducens nerve, the facial (CNVII) and vestibulocochlear (CNVIII) nerves emerge from the pontomedullary sulcus and course toward the internal auditory meatus. Along the rostral lateral medulla emerge in sequence the glossopharyngeal (CNIX), vagus (CNX), and spinal accessory (CNXI) nerves. The hypoglossal (CNXII) nerve exits the sulcus between the pyramid and olive. The margin of the foramen magnum marks the continuation of the medulla as the spinal cord. (Image courtesy of AL Rhoton, Jr.)