The sinuses are air filled spaces within the skull. There are four groups of sinuses:
1. Maxillary sinuses, a paired group located below the eyes and lateral to the nasal cavity.
2. Ethmoid sinuses, typically numbering about 10-15 and arranged in a honeycomb pattern between the eyes.
3. Frontal sinus, a large single or divided cavity above the eyebrows.
4. Sphenoid sinus, a single or divided space located behind the nose, nearly in the center of the skull.
The sinuses are normally lined by a thin layer of tissue called mucosa. Glands within the mucosa produce a secretion called mucous. Mucous helps to moisten and clean the nose, and also helps in the sense of smell. The mucosa cells lining the sinuses have tiny hair-like structures called cilia that sweep the mucous to small openings in the sinus wall.
Under normal circumstances the mucous produced in the sinuses is cleared in a self-cleaning process. However, if the natural openings between the sinuses and the nasal cavity are blocked, the mucous can accumulate in the sinuses. Anything that blocks the outflow of this mucous can lead to sinus pressure, pain, and infections.
The illustration to the left shows a schematic of this cleaning process. The sinus shown here is the maxillary sinus, and its opening is actually up high along the wall. The blue arrow represents the path the mucous from the sinus takes as it passes into the nasal cavity.
Once the mucous reaches the nasal cavity it is then swept back towards the back of the throat and swallowed. The average person actually prduces two to four cups of mucous per day.