The ~sinuses~ are ~air~-filled cavities within the |skull| bones, joined by openings to the main airway inside the |nose|. Four pairs of ~sinuses~ (the frontal, the maxillary, the sphenoidal, the ethmodial) drain into the nasal cavities. The maxillary are the largest ~sinuses~. They extend from the floor of the |orbits| to the roots of the upper |teeth|. The ~sinuses~ are lined with |mucous membrane|, which becomes infected whenever you have a cold. The ~sinuses~ fill with ~mucous~ causing that "stuffed-up" feeling and sometimes pain. ~Sinus~ ~infections~ often develop from colds.
