Blood–brain barrier
Paul Ehrlich, a German doctor, discovered the blood-brain barrier.  He injected dye into mice's bloodstreams.  The dye got to every organ except the brain.  The blood-brain barrier had stopped the dye from getting through to the brain.  Then one of Ehrlich's students injected the dye right into the mice's brains.  This time the brain turned blue, but no other organs did.  The blood-brain barrier had stopped the dye from getting out of the brain into the bloodstream (which would have carried the dye to every other organ in the body).