It was thought for years that the interstitial tissues of the brain served as a barrier to keep damaging substances from reaching the neurons of the brain and cerebrospinal fluid. Instead of a barrier, it would be more accurate to consider it as a sieve or filter through which only molecules of a certain size or smaller can pass.
Most of the molecules of the substances used in chemotherapy are too large to pass through the blood-brain filter, which is why doctors say that chemotherapy doesn't work on brain cancer. Some of the smaller molecules get through, but not the whole suite of drugs intended.
Doctors don't know for sure, but it seems that in order to cross the blood-brain barrier, only molecules less than 800-1000 atomic mass units (amu) in molecular weight can get through. Lipid soluability seems to be another factor which facilitates passing through the blood-brain barrier. Water soluable molecules don't usually penetrate into brain tissue, even when very small. The molecules of essential oils are all not only small, but lipid soluable as well.
In fact, when it comes to essential oils, small molecules (less than 500amu) are what they are made of. That is why they are aromatic. The only way for something to be aromatic is for the molecules to be so small that they readily leap into the air so they can enter our noses and be detected as odor and smell.
That is why oils for cooking or message, such as corn, peanut, sesame seed, safflower, walnut, almond, canola, olive and other oils pressed from seeds are not aromatic. Sure, they have a smell, but you can't smell them across the room in minutes as one can when he opens a bottle of peppermint, hyssop, or cinnamon oil.
Essential oils of every species cross the blood-brain barrier. This makes them uniquely able to address disease, not only from a physical level, but from a more basic fundamental level - that of the emotions which are often the root cause of physical illness.