There are four pairs of sinuses (named for the skull bones in which they are located):

  • Frontal sinuses: The right and left frontal sinuses are located in the center of the forehead (frontal bone) just above each eye.
  • Maxillary sinuses: These are the largest of the sinuses and are located behind the cheekbones near the maxillae, or upper jaws.
  • Sphenoid sinuses: The sphenoid sinuses are located in the sphenoid bone near the optic nerve and the pituitary gland on the side of the skull.
  • Ethmoid sinuses: The ethmoid sinuses are located in the ethmoid bone, which separates the nasal cavity from the brain. These sinuses are not single sacs but a collection of six to 12 small air cells that open independently into the nasal cavity. They are divided into front, middle, and rear groups.

Like the nasal cavity, the sinuses are all lined with mucus. The mucus secretions produced in the sinuses are continually being swept into the nose by the hair-like structures (called ‘cilia’) on the surface of the respiratory membrane.

When they aren’t moistening the air we breathe through our noses, the hollow sinuses act to lighten the bones of the skull. Sinuses also serve as sound-resonance chambers for speech.

The paired and often asymmetrical (not perfectly mirrored) sinuses are small, or rudimentary, at birth but grow as the skull grows. They are fairly well developed by age seven or eight, but don’t reach their maximum size until after puberty. In adults, the sinuses vary considerably in size and shape.

Sinuses often get infections. Sinusitis is inflammation of a sinus caused by a bacterial infection that can follow a viral infection. This causes pus and mucus to accumulate in the sinus. Symptoms can include fever, headache, stuffy nose, and impaired sense of smell.