The<unk>  is one of the four connected fluid-filled cavities within the human brain. These cavities, known collectively as the ventricular system, consist of the left and right lateral<unk> s, the third<unk> , and the<unk> . The<unk>  extends from the cerebral aqueduct (aqueduct of Sylvius) to the obex, and is filled with cerebrospinal fluid (CSF). The<unk>  has a characteristic diamond shape in cross-sections of the human brain. It is located within the pons or in the upper part of the medulla oblongata. CSF entering the<unk>  through the cerebral aqueduct can exit to the subarachnoid space of the spinal cord through two lateral apertures and a single, midline median aperture.
Fourth ventricle
Rhomboid fossa
Middle cerebellar peduncle
Midbrain tegmentum
Diencephalon
Postrhinal cortex
Piriform cortex
Horizontal fissure of cerebellum