A sinus is the hollow part of the sight or more cavities is the bones of the forehead and face. There are two frontal sinuses in the frontal bones in the forehead sad two maxillary sinuses in the cheek bones. The sphenoid and ethmoid sinuses are single or double or multiple, rather than paired. Mastoid sinuses are behind the ears sad connect with them.
All of these cavities except the mastoids have openings into the nasal passage. All are lined with mucous membranes. The mucous membranes connect with those of the nose and sinus secretions drain through the nose.
The sinuses are important because the mucous secretion helps to keep the nose warm sad clean. Much of the air breathed in contains dirt sad germs, which are kept out by the hairs and mucous fluid in the nasal passages.
Without sinus secretions the nose would be too dry and much harmful material would be breathed into the lunge.