An infolding of the This is the outer, most prominent and the toughest of the three different membranes that invest the brain and spinal cor... lining the skull forms a near-complete membrane that separates the lowermost part of the cranial cavity – the That part of the cranial cavity that lies below the tentorium and which contains the brain stem and the cerebellum. – from the remaining volume inside the skull. This tentorium does have an opening within it, through which the upper part of the This is the most primitive part of the brain, connecting the rest of the brain to the body, via the spinal cord. In addi... passes, leaving only a relatively narrow channel through which This clear, water-like fluid forms, for the most part, within the brain, more specifically inside the ventricles of the ... can flow. An important consequence is that the That part of the cranial cavity that lies below the tentorium and which contains the brain stem and the cerebellum. is a relatively isolated part of the cranial cavity and problems can develop here without there being any associated abnormalities supra-tentorially – i.e. above the tentorium.