Paranasal sinuses are air-filled spaces, communicating with the nasal cavity, within the bones of the skull and face. Humans possess a number of paranasal sinuses, divided into subgroups that are named according to the bones within which the sinuses lie. Paranasal sinuses occur in a variety of animals (including most mammals, birds, non-avian dinosaurs, and crocodilians). The biological role of the sinuses is debated, but a number of possible functions have been proposed such as decreasing the relative weight of the front of the skull, and especially the bones of the face; increasing resonance of the voice; and providing a buffer against blows to the face.

It has also been suggested they insulate sensitive structures like dental roots and eyes from rapid temperature fluctuations in the nasal cavity, and also serve to humidify and heat inhaled air. The paranasal sinuses are joined to the nasal cavity via small orifices called ostia. These become blocked easily by allergic inflammation, or by swelling in the nasal lining which occurs with a cold. If this happens, normal drainage of mucus within the sinuses is disrupted, and sinusitis may occur. These conditions may be treated with drugs such as pseudoephedrine, which causes vasoconstriction in the sinuses, reducing inflammation, by traditional techniques of nasal irrigation, or by corticosteroid.

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