Overview

The fourth ventricle connects above to the cerebral aqueduct and continues below as the central canal at the obex. Its floor, the rhomboid fossa, is formed by the back of the pons and the open medulla and is divided by a median sulcus. Lateral to the sulcus lie the swellings over important nuclei:
• the facial colliculus, formed by the abducens nucleus and the genu of the facial nerve;
• the hypoglossal trigone;
• the vagal trigone, over the dorsal motor nucleus of the vagus;
• the vestibular area, laterally;
• the area postrema, at the caudal tip;
• the striae medullares, crossing transversely.
The sulcus limitans separates the medial motor nuclei from the lateral sensory nuclei.

The roof is tent-shaped. Its upper part is the superior medullary velum between the superior cerebellar peduncles. Its lower part is the inferior medullary velum and the tela choroidea, which carry the choroid plexus and are pierced by the median aperture (of Magendie). Long lateral recesses wrap around the brainstem and open as the lateral apertures (of Luschka) at the cerebellopontine angles. Tufts of choroid plexus protrude through them ("Bochdalek's flower baskets"). Through these three openings, CSF passes from the ventricles into the subarachnoid space, mainly the cisterna magna.

Location: Posterior cranial fossa: anterior to the cerebellum, posterior to the pons and upper (open) medulla.

Parts & subdivisions

Floor (rhomboid fossa)
Facial colliculus, hypoglossal and vagal trigones, vestibular area, area postrema, striae medullares; sulcus limitans.
Roof
Superior medullary velum (upper), inferior medullary velum and tela choroidea with choroid plexus (lower); cerebellum behind.
Median aperture (of Magendie)
Midline opening in the lower roof into the cisterna magna.
Lateral apertures (of Luschka)
At the ends of the lateral recesses, opening into the cerebellopontine angle cisterns.
Area postrema
Circumventricular chemoreceptor trigger zone (no BBB) at the caudal floor; triggers vomiting.

Function

Conducts CSF to the subarachnoid space (the only exits from the ventricular system)
Produces CSF (roof choroid plexus)
Its floor holds the cranial nerve nuclei V–XII and the vomiting centre input zone

Clinical significance

Posterior-fossa tumours

Ependymoma (arising from the floor of the fourth ventricle and squeezing through its apertures), medulloblastoma (from the vermis, filling the ventricle) and pilocytic astrocytoma obstruct the fourth ventricle in children. They cause hydrocephalus, morning vomiting and ataxia. Involvement of the facial colliculus produces VI and VII palsies.

Dandy–Walker malformation

Cystic enlargement of the fourth ventricle with hypoplasia of the cerebellar vermis and an enlarged posterior fossa (high tentorium) often causes hydrocephalus in infancy.

Mnemonics

Apertures

"Magendie Median, Luschka Lateral" — one median and two lateral openings.

Test yourself

CSF passes from the third ventricle to the fourth ventricle through the:

Interventricular foramen (of Monro)
Central canal
Median aperture (of Magendie)
Cerebral aqueduct (of Sylvius)

Answer: Cerebral aqueduct (of Sylvius)

The pathway is lateral ventricles → interventricular foramina → third ventricle → cerebral aqueduct (in the midbrain) → fourth ventricle → median and lateral apertures → subarachnoid space. The aqueduct is the narrowest point and the commonest site of congenital obstruction.