It has long been known that there are certain unusual features connected with the interchange of substances between the blood and the brain.
In 1900 Lewandowsky,1 experimenting with sodium ferrocyanide, found a marked difference in the reaction of rabbits to this substance, depending on the mode of administration. If the chemical was introduced into the blood, even in fairly large quantities, no significant reactions were elicited, but if a minute quantity was placed directly into the cerebrospinal fluid, severe nervous disturbances with convulsions followed immediately, sometimes with lethal outcome. It seemed clear that material in the cerebrospinal fluid penetrates to the nerve cells with ease and that the nerve cells display a positive affinity for the ferrocyanide ion. Yet when the substance was introduced into the blood in doses from one to two hundred times greater than those injected in the cerebrospinal fluid, no nervous reactions were
KING LS. THE HEMATOENCEPHALIC BARRIER. Arch NeurPsych. 1939;41(1):51–72. doi:10.1001/archneurpsyc.1939.02270130061003
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