Muscle man illustrations, front and back views. The circles on either side of the head illustrate a medieval conception of the brain's four ventricles. Mid 15th-century. Pseudo-Galen (Claudius), Anathomia; Western Manuscript 290. The four cavities of the human brain are called ventricles. The two largest are the lateral ventricles in the cerebrum; the third ventricle is in the diencephalon of the forebrain between the right and left thalamus; and the fourth ventricle is located at the back of the pons and upper half of the medulla oblongata of the hindbrain. The ventricles are concerned with the production and circulation of cerebrospinal fluid.