Paul Ehrlich had just injected aniline dye--used to color blue jeans--into a rat's bloodstream. For years the immunologist had been working on ways to stain cells so they would be more visible under a microscope, and aniline looked promising. Soon all the animal's muscles, blood vessels and organs were deep indigo. But for some confounding reason the central nervous system--the brain and spinal cord--remained untouched.

Ehrlich's experiment, done at Berlin's Charité hospital in 1885, provided early evidence for the blood-brain barrier--a vital wall that controls which molecules in the bloodstream can enter the brain or nerve pathways. Oxygen, sugars and amino acids are allowed in; most compounds are kept out. As a result, the brain can do its job inside a secure perimeter not available to any other organ. Which is handy, because substances in air, water and food--as well as toxins and even the body's own hormones--can severely impair the brain's functioning. Easy access would quickly lead to mental chaos.

This article was originally published with the title Crossing the Barrier.

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