The sinuses are a connected system of hollow cavities in the skull. The largest sinus cavities are around an inch across; others are much smaller. The sinus cavities include:
• The maxillary sinuses (the largest), in the cheekbones.
• The frontal sinuses, in the low-center of the forehead.
• The ethmoid sinuses, between the eyes, at the nasal bridge.
• The sphenoid sinuses, in bones behind the nasal cavity.
The sinuses are lined with soft, pink tissue called mucosa. Normally, the sinuses are empty except for a thin layer of mucus.
The inside of the nose has ridges called turbinates. Normally these structures help humidify and filter air. The nose is divided in the center by a thin wall, called the septum. Most of the sinuses drain into the nose through a small channel or drainage pathway called the middle meatus.
The purpose of the sinuses is unclear. One theory is that sinuses help humidify the air we breathe in; another is that they enhance our voices.