There are hollow fluid-filled cavities known as ventricles deep inside the human brain. There are two lateral ventricles, one in each cerebral hemisphere, and each has three different projections — the anterior, inferior, and posterior horns visible from the lateral side. A thin membrane called the septum pellucidum separates the two lateral ventricles. The slender third ventricle in the diencephalon is connected to each lateral ventricle via a channel called the interventricular foramen. The cerebral aqueduct is posterior to the third ventricle, a canal-like structure that connects it to the fourth ventricle between the pons and the anterior surface of the cerebellum.

The brain ventricles are filled with cerebrospinal fluid (CSF), produced and filtered from a network of blood capillaries called the choroid plexus. These capillaries are lined by tightly joined ependymal cells that secrete CSF into the ventricles. The CSF then drains into the central canal of the spinal cord and surrounds the brain. The CSF mainly provides mechanical protection to the central nervous system, enables optimal neural signaling, and acts as a medium for nutrient exchange.