“…Although it is clear that a 'blood-brain barrier' exists (Friedemann, 1942;Krogh, 1946;Tschirgi, 1952;Manery, 1954;Bakay, 1956;Dobbing, 1956;Davson, 1957), there is considerable confusion regarding its nature and site. It is possible that the barrier is related to capillary walls (Rodriguez, 1955) although histologically there is nothing to distinguish these capillaries from those elsewhere in the body (Patek, 1944;Woollam & Millen, 1954 (Tschirgi, 1952;Mayer & Bain, 1956) or even the ground substance between nerve cells (Hess, 1953(Hess, , 1955Woollam & Millen, 1954). It is clear, however, that the ground substance is relatively unimportant, because substances unable to pass the blood-brain barrier are quite effective when applied locally within the nervous system.…”