A nasal sinus is a hollow, air-filled cavity that links the nostrils and the nasal passage. The human head has 4 pairs of sinuses: the frontal sinuses (in the forehead), maxillary sinuses(behind the cheeks), ethmoid sinuses(between the eyes) and sphenoid sinuses (deep behind the ethmoids). Sinuses are covered with a mucous layer and cells that defend against viruses and bacteria. They protect the skull, reduce its weight and allow the voice to make sound within it.
Attribution: Straight Talk Foundation