The ventricular system consists of lateral ventricles, third ventricle, cerebral aqueduct, fourth ventricle, and spinal canal and arachnoid granulations in the dural sinuses. The central nervous system is surrounded by CSF. CSF is in the subarachnoid space which surrounds the brain and is produced in the choroid plexus in the lateral ventricles and IV ventricles. The extracellular space in the brain surrounds every cell in the CNS. It permits communication between cells and diffusion of substances including viruses in between cells in the CNS. The CSF is formed primarily by the choroid plexus in the lateral ventricle and third ventricle. It flows through the four ventricles, from the lateral ventricles through the intraventricular foramen of Monro into the third ventricle and then into the IV ventricle, leaving the brain through the central canal in the spinal cord or into the subarachnoid space in the meninges, and is absorbed into the venous circulation via the arachnoid granulations in the dural sinuses in the