a. CSF is produced by the choroid plexus within the ventricles.
b. CSF flows from the lateral ventricles and third ventricle into the cerebral aqueduct and then into the fourth ventricle. A relatively small amount of CSF from the central canal of the spinal cord travels to the fourth ventricle as well. Most of the CSF in the fourth ventricle flows into the subarachnoid space by passing through openings in its membranous roof, either the paired lateral apertures or the single median aperture. CSF flows through the subarachnoid space surrounding the brain and spinal cord.
c. As additional CSF is incorporated into the subarachnoid space, the hydrostatic pressure on the CSF within the arachnoid villi forces the CSF into the dural venous sinuses, allowing excess CSF to be released into the venous bloodstream without allowing any venous blood to enter the subarachnoid space.
The surgeon must cut through the skin, the external periosteum, the occipital bone of the cranium, the dura mater (first the periosteal layer and then the meningeal layer), the arachnoid, the pia mater, and finally a glial cell covering before reaching the cerebral cortex.