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. The first branch of the basilar artery is typically the anterior inferior cerebellar artery followed by pontine arteries that radiate onto the pons surface. Deep to this arterial tree are venous channels draining the pons, with the transverse pontine vein in view here. Emerging from the ventral pons near the pontomedullary sulcus 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 in company with the labyrinthine artery, a branch of the anterior inferior cerebellar artery. 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 medullary pyramid and olive. (Image courtesy of AL Rhoton, Jr.)