"Periaqueductal grey matter is a column of grey matter that surrounds the cerebral aqueduct of Sylvius in the midbrain. The periaqueductal grey matter extends from the superior border of the midbrain (which forms the posterior aspect of the floor of the third ventricle), inferiorly towards the fourth ventricle and the superior medullary velum. The mesencephalic nucleus and the mesencephalic root of the trigeminal nerve are found lateral to the periaqueductal grey matter. The oculomotor nucleus and the Edinger–Westphal nucleus are found anterior to the periaqueductal grey matter."