Picture of Sinuses
The sinuses are an air-filled cavity in a dense portion of a skull bone. They actually decrease the weight of the skull. The sinuses are formed in four right-left pairs. The frontal sinuses are positioned behind the forehead, while the maxillary sinuses are behind the cheeks. The sphenoid and ethmoid sinuses are deeper in the skull behind the eyes and maxillary sinuses. The sinuses are lined by mucous-secreting cells. Air enters the sinuses through small opening in bone called ostia. If an ostium is blocked, air cannot pass into the sinus and likewise mucous cannot drain out.
Common conditions that affect the sinuses include sinusitus (sinus infection) and upper respiratory infections
Text: MedicineNet, Inc.