The four ventricles in the brain form a connected fluid-filled cavity system. The roof of each ventricle is thin and contains no neurons. Each roof does, however, have a network of projecting capillaries called a choroid plexus. These plexuses, together with the ependymal cells that line the surface, continuously secrete cerebrospinal fluid into the ventricles of the brain. The cerebrospinal fluid (CFS) is an excellent shock absorber and helps to remove waste products from the brain cells. The entire central nervous system contains about 125 ml of cerebrospinal fluid. It is a clear, colorless fluid resembling a partially deproteinated plasma, but its chemical composition is very complex since a major function of the choroid plexes is to protect the CNS neurons from the more toxic substances found in the blood.
The colored areas in slide 1 show the four ventricles. The two lateral ventricles are shown side-by-side in the cerebrum (two-shaded orange colored) with the third ventricle (blue) in the diencephalon. Finally, the fourth ventricle (red) is located in the brainstem at the base of the cerebellum.