What are the Sinuses/paranasal sinuses?

The paranasal (i.e. next to the nasal cavity) sinuses are hollow, air filled chambers within the facial bones and skull. There are four sinuses located on each side of the nasal cavity: frontal, ethmoid, maxillary, and sphenoid. The frontal, maxillary and sphenoid sinuses are single chamber sinuses located above, below, and behind the eyes respectively. The ethmoid sinuses are a collection of honeycomb air cells located between the eyes.

What is the function of the paranasal sinuses?

The primary role of the nasal cavity is to warm, humidify and clean inspired air before it reaches the lungs. The paranasal sinuses are an extension of the nasal cavity. While the exact purpose of the paranasal sinuses is unclear, it is theorized that they play a role in: decreasing weight in the head, creating resonance to the voice, helping to filter air, keeping the nose moist, and helping the body's immune system.