<P> CSF occupies the subarachnoid space (between the arachnoid mater and the pia mater) and the ventricular system around and inside the brain and spinal cord . It fills the ventricles of the brain, cisterns, and sulci, as well as the central canal of the spinal cord . There is also a connection from the subarachnoid space to the bony labyrinth of the inner ear via the perilymphatic duct where the perilymph is continuous with the cerebrospinal fluid . </P> <P> A sample of CSF can be taken via lumbar puncture . This can reveal the intracranial pressure, as well as indicate diseases including infections of the brain or its surrounding meninges . Although noted by Hippocrates, it was only in the 18th century that Emanuel Swedenborg is credited with its rediscovery, and as late as 1914 that Harvey W. Cushing demonstrated CSF was secreted by the choroid plexus . </P> <P> There is about 125 - 150 mL of CSF at any one time . This CSF circulates within the ventricular system of the brain . The ventricles are a series of cavities filled with CSF . The majority of CSF is produced from within the two lateral ventricles . From here, CSF passes through the interventricular foramina to the third ventricle, then the cerebral aqueduct to the fourth ventricle . From the fourth ventricle, the fluid passes into the subarachnoid space through four openings--the central canal of the spinal cord, the median aperture, and the two lateral apertures . CSF is present within the subarachnoid space, which covers the brain, spinal cord, and stretches below the end of the spinal cord to the sacrum . There is a connection from the subarachnoid space to the bony labyrinth of the inner ear making the cerebrospinal fluid continuous with the perilymph in 93% of people . </P> <P> CSF moves in a single outward direction from the ventricles, but multidirectionally in the subarachnoid space . Fluid movement is pulsatile, matching the pressure waves generated in blood vessels by the beating of the heart . Some authors dispute this, posing that there is no unidirectional CSF circulation, but cardiac cycle - dependent bi-directional systolic - diastolic to - and - fro cranio - spinal CSF movements . </P>

Where is most of the cerebrospinal fluid found