Cerebral Aqueduct

The cerebral aqueduct, the extension of the ventricle through the mesencephalon, communicates rostrally with the third ventricle and caudally with the fourth ventricle. This midline channel is about 1.5 mm in diameter in adults and contains no choroid plexus. Its narrow diameter makes it especially susceptible to occlusion. For example, cellular debris in the ventricular system (from infections or hemorrhage) may clog the aqueduct. Tumors in the area of the midbrain (such as pinealoma) may compress the midbrain and occlude the aqueduct. The result is a blockage of CSF flow and enlargement of the third and lateral ventricles at the expense of the surrounding brain tissue. The cerebral aqueduct is surrounded on all sides by a sleeve of gray matter that contains primarily small neurons; this is the periaqueductal gray or central gray.