The fourth ventricle 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 ventricles, the third ventricle, and the fourth ventricle. The fourth ventricle extends from the cerebral aqueduct (aqueduct of Sylvius) to the obex, and is filled with cerebrospinal fluid (CSF). The fourth ventricle 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 fourth ventricle through the cerebral aqueduct can exit to the subarachnoid space of the spinal cord through two lateral apertures and a single, midline median aperture. Boundaries The fourth ventricle has a roof at its upper (posterior) surface and a floor at its lower (anterior) surface, and side walls formed by the cerebellar peduncles (nerve bundles joining the structure on the posterior side of the ventricle to the structures on the anterior side). The caudal tip of the fourth ventricle - where it becomes the central canal - is known as the obex; the obex is also a marker for the level of the foramen magnum of the skull and therefore is a marker for the imaginary dividing line between the medulla and spinal cord The superior portion of the roof (i.e. of the posterior edge) is a thin lamina - the superior medullary velum - connecting the left and right superior cerebellar peduncles together. The inferior portion of the roof - the inferior medullary velum - has a tricorn cross section, directed caudally and laterally, and is formed by the Cerebellum directly. Near each of the 3 corners of the inferior roof is an opening into the cisterna magna, the caudal opening being the foramen Magendie, while the lateral openings are the foramina of Luschka. The roof rises (i.e. posteriorly) to a peak, known as the fastigium (Latin for "summit"); the fastigial nucleus lies immediately above the roof of the fourth ventricle, in the cerebellum. The floor (i.e. the anterior edge) of the fourth ventricle constitutes the rhomboid fossa, and comprises a number of general features. A sulcus - the median sulcus - extends the length of the ventricle (from the cerebral aqueduct of the midbrain to the central canal of the spinal cord), dividing the floor into right and left halves. Each half is further divided by