The paranasal sinuses are eight (four pairs) air-filled spaces, or sinuses, within the bones of the skull and face. These are divided into subgroups that are named according to which bones they lie under;

• the maxilliary sinuses, also called the antra, are under the eyes, in the upper jawbone.
• the frontal sinuses, over the eyes, in the bone of the forehead.
• the ethmoid sinuses, between the nose and the eyes, backwards into the skull.
• the sphenoid sinuses, in the centre of the skull base.

The function of the sinuses is not entirely clear, but they may fulfill any or all of the following:

• Decreasing the relative weight of the front of the skull
• Helping to warm and humidify inhaled air before it reaches the lungs
• Increasing resonance of the voice
• Providing a buffer against blows to the face

The paranasal sinuses are joined to the nasal cavity via small orifices called ostia. These become blocked relatively 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.