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.