That part of the cranial cavity that lies below the tentoriumAn infolding of the dura lining the skull forms a near-complete membrane that separates the lowermost part of the cranial cavity – the posterior fossa – from the remaining volume inside the skull. This tentorium does have an opening within it, through which the upper part of the brain stem passes, leaving only a relatively narrow channel through which cerebrospinal fluid can flow. An important consequence is that the posterior fossa 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. and which contains the brain stemThis is the most primitive part of the brain, connecting the rest of the brain to the body, via the spinal cord. In addition to feeding signals to and from such “higher centres”, the brain stem contains a number of important control centres, regulating vital functions such as breathing and blood pressure. and the cerebellumThis word means the “little brain” and refers to a large part of the brain that sits in lowermost part of the skull, at the back of the head, immediately above the top of the spine. This part of the skull is known as the posterior fossa..