Cerebral Aqueduct

The structure indicated by the arrow is the cerebral aqueduct. The cerebral aqueduct (of Sylvius) forms part of the ventricular system of the brain which connects the third ventricle located in the diencephalon, to the fourth ventricle located in the brainstem. The ventricular system of the brain is made up of four ventricles:

• 2 lateral ventricles (found in the telencephalon)
• Third  ventricle (found in the diencephalon)
• Fourth ventricle (found in the rhombencephalon)

The interventricular foramen (foramen of Monro) connects the lateral ventricles to the third ventricle. The cerebral aqueduct connects the third ventricle to the fourth ventricle. If the cerebral aqueduct becomes obstructed, a condition called obstructive hydrocephalus may develop, resulting in the dilatation of the lateral and third ventricles.