The normal direction of flow of the cerebrospinal fluid has been indicated and its main points of exit from the subarachnoid space have been demonstrated by Weed1 in 1914 and 1923, using careful physiologic controls. By the method of introducing isotonic potassium ferrocyanide and iron ammonium citrate solution into the subarachnoid space under pressures closely approaching the normal, he showed that the chief channels of escape of the cerebrospinal fluid were the arachnoid villi. If, however, a hypertonic solution of sodium chloride were previously injected into a vein, accessory paths of elimination were opened (probably due to a reversal in direction of flow) along the perivascular spaces and through the ependymal cells lining the ventricles, into the capillaries and venules of the brain substance.
Foley,2 using Weed's method with modifications, substantiated these observations, but also showed the presence of Prussian blue granules within the vessels of the choroid
FORBES HS, FREMONT-SMITH F, WOLFF HG. RESORPTION OF CEREBROSPINAL FLUID THROUGH THE CHOROID PLEXUS. Arch NeurPsych. 1928;19(1):73-77. doi:10.1001/archneurpsyc.1928.02210070100004