The brain has a few small areas with no blood brain barrier, which act as different types of chemical sensors, for things like balancing salt levels, temperature, detecting toxins. This is just a technical book on those areas that I downloaded and listened to on the bus, what else is there to say? There's not as much on the area postrema (the toxin detecting well in the brainstem, involved in nausea) as I was looking for with my current interest in chemoreception