Paranasal sinuses are a group of four paired air-filled spaces that surround the nasal cavity. The maxillary sinuses are located below the eyes; the frontal sinuses are above the eyes; the ethmoidal sinuses are between the eyes, and the sphenoidal sinuses are behind the eyes. The sinuses are named according to the bones composing them, namely the frontal, maxillary, ethmoid and sphenoid bones. The evolutionary function of the sinuses is still partly debated.