Sinus is a generic term derived from latin – it means ‘fold’ or ‘pocket’. What ‘sinuses’ generally refer to are the paranasal sinuses, a collection of air-filled cavities within the bones of the skull & face. The total volume of these cavities varies from person to person but an average is about 70ml.
Why they’re there is up for debate but weight reduction for the skull, heating/humidification of inhaled air, improved vocal resonance and temperature insulation for teeth are possibilities.
Those located above & behind the eyes are called the frontal sinuses, the biggest ones found under the eyes at the front of the face are called the maxillary sinuses. The last two groups are more centrally located – the ethmoid sinuses are a group of several air cells found right between the eyes and the sphenoid sinus is right up the back there, near the pituitary gland.
Paranasal sinuses are connected to the nasal cavity via channels called ostia. The internal surfaces of the sinuses are lined with cilia to move goo around, but they’re particularly concentrated in the ostia to aid drainage into the nasal cavity. Lovely stuff. When inflammation or some other obstruction blocks the drainage, a sinus cavity can end up under pressure which is pretty painful. That would be sinusitis.
And those are today’s sinus facts! I have other sinus facts, but it gets a bit yuck from here on in.