Sinuses are air-filled sacs (empty spaces) on either side of the nasal cavity that filter and clean the air breathed through the nose and lighten the bones of the skull. There are four paired sinuses ...

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There are 4 sets of sinuses ordinarily present in humans, but congenital absence or hypoplasia of one or more sinus cavities is not uncommon. The maxillary and ethmoidal sinuses form during the third ...