Which brain structure connects the third and fourth ventricles?

The cerebral aqueduct connects the third and fourth ventricles.

Also called the aqueduct of Sylvius, the cerebral aqueduct is a narrow channel in the midbrain. It allows cerebrospinal fluid to flow from the third ventricle, located in the diencephalon, to the fourth ventricle, which lies between the brainstem and cerebellum.

Because it is so narrow, the cerebral aqueduct is a frequent site of obstruction. Blockage can cause cerebrospinal fluid to accumulate upstream, producing non-communicating hydrocephalus and increasing pressure within the skull. Congenital narrowing is one possible cause, although obstruction can have other origins.

The cerebral aqueduct is not the same as the interventricular foramina, which connect each lateral ventricle to the third ventricle. Nor is it the central canal, a narrower continuation associated with the spinal cord. These channels are parts of one connected ventricular system.