arachnoid villi

tufted prolongations of pia-arachnoid that protrude through the meningeal layer of the dura mater and have a thin limiting membrane; collections of arachnoid villus form arachnoid granulations that lie in venous lacunae at the margin of the superior sagittal sinus; the spongy tissue of the a. villus contains tubules that serve as one-way valves for transfer of cerebrospinal fluid from the subarachnoid space to the venous system. Both a. villus and the granulations formed from them are major sites of fluid transfer. See Also: arachnoid granulations, under granulation.