 Any substance entering the central nervous system from the blood or cerebrospinal fluid (CSF) must cross the glia limitans.The two different classifications of glial limiting membrane, the glia limitans perivascularis and the glia limitans superficialis, have nearly identical structures, however, they can be distinguished from each other by their location within the brain.The glia limitans perivascularis abuts the perivascular space surrounding the parenchymal blood vessels and functions as a supportive constituent of the blood–brain barrier.