Cerebral aqueduct

The mesencephalic duct, also known as the aqueduct of Silvius or the cerebral aqueduct, contains cerebrospinal fluid (CSF), is within the mesencephalon (or midbrain) and connects the third ventricle in the thalamus (or diencephalon) to the fourth ventricle, which is between the pons and cerebellum.

A blockage in this duct is a cause of hydrocephalus.