part of nervous system multi-tissue structure perineurium Some authors use this term only to refer to the double layer of cells covering the CNS (e.g. Hartenstein and Jan, 1992), while other authors use it more broadly to include the covering of nerves (e.g. Banerjee et al., 2006). This ontology uses the term in its broad sense. 'Blood-brain barrier' may refer to this (e.g. Limmer et al., 2014) or the subperineurial glial sheath alone (e.g. Morante et al., 2013). fly_anatomy.ontology blood-brain barrier VFB:FBbt_00007090 The outer two layers of glial cells that cover the central nervous system and the nerves. The outer layer is the perineurial glial sheath (sometimes referred to as the perineurial sheath outer layer); the inner layer is the subperineurial glial sheath (sometimes referred to as the perineurial sheath inner layer). FBbt:00007090