Paranasal sinuses are parts of thenasal cavity.[1] They are usually counted to be part of therespiratory system. They aremucous membranes, which are mostly insidebones. These bones are then filled with air, which makes the skull look bigger, without adding much extra weight. Prenasal cavities can be found inmammals, mostbirds,dinosaurs, andcrocodilians.
Since sinuses are in virtually all landvertebrates, they must have a function. They are covered with amucosa,epidermal cells which are moist and makemucus. This moistens and warms the air on its way into thelungs. The mucus also picks up particles which come in with the air. A variety of possible functions have been proposed:
