part of hindbrain fourth ventricle BTO:0003426 BrendaTissueOBO The fourth ventricle is one of the four connected fluid-filled cavities within the human brain. 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. ventricle of hindbrain ventricle of rhombencephalon ventriculus quartus