and you thought the blood-brain barrier was the ultimate in impermeable coverings...
The area postrema is a portion of the medulla oblongata where the blood-brain barrier is weak. As a result, substances circulating in the blood pass freely into the neurons in this area, loading this abused portion of your brain with all sorts of toxic substances.
Do not fear! Before you shelve your ideas about the amazing wonder of the human body, know this: without your area postrema, you would never vomit. That's right! What initially seemed like a failure is, in fact, a means to detect poisons, and cause them to be expelled from your body before they kill you. Just think of how many times you might have died of alcohol poisoning were it not for the stimulation of your area postrema!
Hooray for the area postrema! It keeps us all safe from harm!