Your maxillary sinuses are on either side of your nasal cavity, back there in your cheekbones above your upper canines and molars. And — as shown that 17th-century medical story — the roots of those teeth sometimes poke up into the maxillary sinuses.
The frontal sinuses are in your forehead, just above your eyebrows, and your ethmoidal sinuses are a couple of little pockets between your eyes. The sphenoidal sinuses hang out way back in there behind your nose.
They're all linked by tiny channels collectively known as the osteomeatal complex, which connects them with the nasal passages. Their special lining, fittingly called mucosa, produces mucus [source: Jacobs]. Gravity drains the mucus into the nasal cavity.
Some researchers think sinuses are just accidental byproducts of evolution that have no real function. Sinuses expand and change form as we grow, so other specialists have claimed they actually take shape as the result of the chewing process, which creates enough pressure to open up little caves in our skulls. Still others theorize that atmospheric pressure does the trick as we develop after birth [source: Kurbel]. Our sinuses await an answer to these existential questions. To add to those unknowns, our sinuses aren't even symmetrical. Each cavity grows to a unique size and shape, making them as individual as our fingerprints.
On top of figuring out how they wind up there, we're also still working on why they're there. Two thousand years ago or so, the famed Greek physician Galen suggested sinuses existed to reduce weight in the head. Later theorists proposed that they help regulate the temperature and humidity of the air we breathe in, or that they relieve pressure on the nasal cavity when sneezing, or that they're needed for mucus production, or that they help with smelling, or that they're resonating chambers for our voices. Those theories sound neat, but they've all been refuted [source: Bergler]. Since we don't really know why they're there, we also don't know whether the different sinuses serve different functions.
One of the most controversial suggestions belongs to the highly disputed theory known as the "aquatic ape" idea of human evolution. According to this idea, our ancestors didn't evolve into modern humans on the African savannah as the conventional theory goes, but rather on the shores of rivers, lakes and oceans. If this were true, it could explain some of the peculiarities that set us apart from other apes and link us more closely to sea creatures: notably, our lack of fur, our big brains, our subcutaneous fat and our sinuses. Having sinuses makes perfect sense, according to the concept, because skulls full of air pockets are more buoyant than skulls without air pockets [source: McKie].
For whatever reason they evolved, our sinuses seemingly aren't just useless skull dugouts. In 1995 it was discovered that they produce large quantities of the gas nitric oxide. Nitric oxide can increase blood pressure and has antibacterial properties. In other words, your nose gas probably helps inhibit the growth of pathogens in the nasal cavity [source: Lundberg]. Mind you, current function shouldn't be confused with origin. Just because sinuses produce useful nitric oxide, that doesn't mean they evolved for that purpose. The gas production could be an incidental byproduct of a different process.
But if the sinuses help our immune system, we all know that system doesn't always succeed, and sometimes those pockets in our skulls are filled with more than friendly gases.