What is Sinuses?
The paranasal sinuses are air-filled pockets located within the bones of the face and around the nasal cavity. Each sinus is name for the bone in which it is located:

Maxillary (one sinus located in each cheek)
Ethmoid (approximately 6-12 small sinuses per side, located between the eyes).
Frontal (one sinus per side, located in the forehead)
Sphenoid (one sinus per side, located behind the ethmoid sinuses, near the middle of the skull)

Each of these pockets has an opening that connects to the nose. This opening is called an ostium. The paranasal sinuses are covered with a special lining (or epithelium). The lining secretes mucus, a complex substance that keeps the nose and sinuses moist. The sinus epithelium is ciliated; that is, each cell on its surface has a cilium, which is a relatively long structure that has the capacity to push sinus mucus. This movement of mucus (which is known as mucociliary clearance) is not random; rather, it is programmed so that the mucus moves along in a specific pattern. The sinus do not ‘drain’ by gravity-it is an active process.