Nasal cavity and paranasal sinuses. Cutaway illustrations of a side view (left) and a frontal view (right) of the nasal cavity and the facial bones, showing the paranasal sinuses. Shown here are the frontal sinuses (in the forehead), the ethmoid sinuses (between the eyes), the maxillary sinuses (in the cheek bones), and the sphenoid sinuses (behind the ethmoid sinuses). These are air spaces in the facial bones around the eyes and nose. An infection of the membranes in these spaces (sinusitis) causes an increase in mucus production, which can block the sinuses, causing internal pressure.