Blood–brain barrier

The blood-brain barrier is a physical barrier in the circulatory system that stops many substances from travelling into the brain and central nervous system.

The fact that such a barrier existing was first noticed in experiements by Paul Ehrlich in the late 19th century. Ehrlich was a bacteriologist and was study staining (used for many studies to make fine structures visible). Some of these dyes, notably the aniline dyes that were then popular, would stain all of the organs of an animal except the brain when injected. At the time Ehrlich attributed this to the brain simply not picking up as much of the dye.

However in a later experiment in 1913, Edwin Goldmann (one of Ehrlich's students) injected the dye into the spine directly. He found that in this case the brain would become dyed, but the rest of the body remained dye-free. This clearly demonstrated the existance of some sort of barrier between the two sections of the body.