A cycle of cerebrospinal fluid: supporting evidence and theoretical considerations.

An entrenched theory of cerebrospinal fluid (CSF) absorption by the arachnoid villi fails to explain observations regarding the movement of substances in the fluid. Experiments that demonstrated the arachnoid villi as the site of absorption were based on non-physiologic methods. CSF does not flow through the arachnoid villi, because villi require bulk flow and bulk flow of CSF does not exist. CSF is transported through the choroid fissure and recycled through the tela choroidae by the choroid plexus, with reentry into the ventricular system. Observed failures of the effete arachnoid villus theory can be readily explained by the cycling theory. A complete cycle of CSF is suggested to pace the 90 to 100-minute ultradian rhythms found in human physiology.