The brain and spinal cord are surrounded by three connective tissue membranes, the meninges. The pia mater and arachnoid mater are together called the leptomeninges. Trauma which shears major vessels going from the dura mater into the cranial bone causes bleeding into the extradural space that opens up between meninges and skull. Cerebrospinal fluid (CSF) is actively secreted by choroid plexuses situated in the lateral, third, and fourth ventricles. Flow of CSF is from the lateral ventricles through the foramen of Munro into the third ventricle, and then through the aqueduct of Sylvius into the fourth ventricle. The lumbar cistern is the target for sampling CSF because there is no risk of damage to the cord. The ability of the choroid plexus to absorb materials from the CSF means that it acts as an excretory organ for the brain. It scavenges choline, dopamine and serotonin metabolites, urea, creatinine, and K+, dumping them into the blood.