Aqueduct of Sylvius

The aqueduct of Sylvius represents the communication between the third and fourth ventricles. It is 15 to 18 mm long and 1 to 2 mm in diameter. Its shape is like the neck of a swan. Posteriorly, its relations are, from superiorly to inferiorly, the posterior commissure, the superior and inferior colliculi and the anterior medullary velum. Its floor is the midbrain, and contains the nuclei of the third and fourth cranial nerves (Fig. 153).