Neurothelial cell layer sits inferior to dura mater and superior to arachnoid mater. It acts as immunological tissue and shifting layer for pulsating CNS against rigid dura. Prox1-EGFP staining images known neurothelial layer, not new subarachnoid layer. Arachnoid blood vessels lie inferior to neurothelial layer inside subarachnoid space. Subdural fluorophore injection tears neurothelial layer creating artificial subdural space. Subarachnoid fluorophore stays inferior to torn layer. Neurothelium explains CNS lymphatic drainage via dural lymphatics. It clarifies subdural space formation during surgery. Chronic subdural hematomas grow in inflammatory mesothelial cavity without re-bleeding. Subdural effusions develop with meningitides and resolve with inflammation. Burr-hole drainage fails without negative pressure like pleural effusions. Subdural empyemas mirror pleural empyemas. Mesothelium originates from neural tube enfolding in mesoderm. High-resolution OCT needed to image neurothelial layer position. Møllgård's SLYM misidentifies neurothelial cells inside subarachnoid space.