The =fourth ventricle= (Ventriculus quartus) is the cavity of the
rhombencephalon; it communicates with the central canal of the spinal
cord behind, and through the aqueduct with the third ventricle in front.
It is somewhat rhomboid in outline, elongated from before backward and
narrowest behind. It is lined completely by an epithelium (Ependyma) and
contains a small amount of fluid.
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