The mechanism of drainage of the cerebrospinal fluid

As there is a continuous production of cerebrospinal fluid (CSF) primarily by the choroid plexuses of the cerebral ventricles, there must be a drainage mechanism by which this fluid is returned to the blood. Most workers in the field agree that the site of this reabsorption is in the arachnoidal villi which protrude into the venous sinuses draining the brain (Davson, 1967). There is, however, still a controversy over the mechanism by which substances traverse these villi so as to reach the venous blood. On the one hand there are those who have injected substances into the CSF and found these substances to appear in the blood often quite rapidly (Sprong, 1934; Courtice and Simmonds, 1951; Simmonds, 1952; Prockop, Schanker and Brodie, 1962), who have perfused the cerebral ventricular system with artificial CSFs of different colloid osmotic pressures (Davson, Hollingsworth and Segal, 1970), or who have perfused isolated arachnoid villi with particles of varying sizes (Welch and Friedman, 1960; Welch and Pollay, 1961). In each case they have reached the conclusion that there must exist some channels or other open communica-tion between the cerebrospinal fluid and the venous blood. On the other hand