@KaBoom - The brain is a very complex organ. I remember reading somewhere that there is still a lot scientists don't know about the brain. I feel sure that as new discoveries are made they will find new ways to get medications to the brain.
I see that the subarachnoid space contains blood vessels and spinal fluids that help cushion the brain. I think it's important to keep in mind that even though we have this cushion the brain is still fairly delicate. And once you lose brain cells they don't come back!