
The concept of a barrier between the blood and the brain arose in the late 19th century when the German bacteriologist Paul Ehrlich observed that certain dyes administered intravenously to small animals stained all of the organs except the brain. Ehrlich interpreted this to mean that the brain had a lower affinity for the dye than the other tissues. In subsequent experiments, one of Ehrlich's students injected a blue dye directly into the cerebrospinal fluid of rabbits and dogs. The dye readily stained the entire brain, but did not enter the bloodstream to stain the other internal organs.