Skin, then bone of skill (3 layers of membrane protect the brain: meninges) Outer most layer aka dura mater- leather like tissue that follow outlines of skull bones. Then, arachnoid layer- more delicate layer, looks like spider web. Innermost layer, aka pia mater- nearly transparent sticks closely to the outside of the brain (all cover the brain and spinal cord. Only dura and pia, cover exit, called the peripheral nervous system nerves. ) Cerebrospinal fluid (csf) circulates in the hollow spaces of brain aka ventricles. Produced by the choroid plexus- using nearby blood supply within ventricles. Csf circulates through central canal of spinal cord and 4 in brain. New csf made 3 times daily, old absorbed into blood at top of head. Never in direct contact with blood, but composition important for diagnosing diseases aka spinal tap (fluid from subarachnoid space) Hydrocephalus- water on the brain, bc of narrow sections in circulation system, occurs blockages.