Tentorium

An infolding of the duraThis is the outer, most prominent and the toughest of the three different membranes that invest the brain and spinal cord. It also forms the lining of the cranial cavity and the spinal canal. Looked at as a whole, the dura can be envisaged as a balloon with a long sausage skin hanging below it. Within the dura the brain and spinal cord are bathed in cerebrospinal fluid. lining the skull forms a near-complete membrane that separates the lowermost part of the cranial cavity – the posterior fossaThat 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 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. passes, leaving only a relatively narrow channel through which cerebrospinal fluidThis clear, water-like fluid forms, for the most part, within the brain, more specifically inside the ventricles of the brain. Blood flows through structures inside the ventricles, called the choroid plexuses, and clear fluid is produced as a result, passing into the ventricle. This process is akin to blood flowing through the kidneys and urine being produced as a result. An important difference is that, unlike urine, the cerebrospinal fluid is not passed out of the body periodically. Instead, the fluid is re-absorbed back into the blood stream, mostly via the arachnoid granulations, into the cerebral venous sinuses. In the course of a 24-hour period, most healthy adults will produce about 500mls (or a pint) of cerebrospinal fluid, although at... can flow. An important consequence is that the posterior fossaThat 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.