The fourth ventricle is one of the components of the ventricular system in the brain, along with the lateral and third ventricles. It extends from the cerebral aqueduct (of Sylvius) rostrally to the obex caudally and is filled with CSF.
CSF enters the ventricle via the cerebral aqueduct and leaves via one of four routes:
- via the obex and into the central canal
- via the median aperture (of Magendie) into the cisterna magna
- via one of the two lateral apertures (of Luschka) into the cerebellopontine cistern
It is characteristically diamond-shaped in sagittal cross-section and located dorsal to the pons or upper part of the medulla oblongata. The roof, located dorsally is formed by the superior medullary velum and inferior medullary velum overlying the cerebellum. The floor, located ventrally, is formed by the rhomboid fossa, a diamond-shaped depression on the dorsal surface of the pons and upper half of the medulla. The sidewalls are formed by the vela and cerebellar peduncles. It is widest at the level of the pontomedullary junction. The obex is the most caudal tip of the fourth ventricle.
The fourth ventricle contains choroid plexus along its roof along the tela choroidea which may protrude out the lateral foramina of Luschka.