c) The meninges are the system of membranes that envelop the central nervous system, its main function is to protect the central nervous system. The blood-brain barrier is a cellular and metabolic barrier located at the capillaries in the brain. It alters permeability, restricting the passage of some chemical substances and microscopic objects from the bloodstream into neural tissue, while other substances are able to pass through to the brain. Astrocytes permit ions to travel from the brain to the blood. Solubility permits caffeine, nicotine, and alcohol to pass through the blood brain barrier because only those items that are soluble in fat are able to clear the barrier.