Fourth ventricle

Ventriculus quartus

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What it is
A tent-shaped cavity between the pons and upper medulla in front (its diamond-shaped floor, the rhomboid fossa) and the cerebellum behind (the roof peaks at the fastigium); lateral recesses reach round the brainstem.
Function
Lets CSF escape from the ventricles into the subarachnoid space through the median aperture (Magendie) and two lateral apertures (Luschka).
Size
About 3 cm long and 1.5-2 cm wide at the lateral recesses
Clinical note
Its floor holds the facial colliculus, vagal and hypoglossal triangles and the area postrema, the vomiting centre that senses toxins in the blood.

Nearby nervous structures Accessory nerve root (CN XI), Cerebellar white matter (arbor vitae), Facial nerve root (CN VII), Glossopharyngeal nerve root (CN IX), Medulla oblongata, Pons, Trigeminal nerve root (CN V), Trochlear nerve root (CN IV), Vagus nerve root (CN X), Vestibulocochlear nerve root (CN VIII)