There are four pairs of paranasal sinuses, the frontal sinuses are located above the eyes, in the forehead bone. The maxillary sinuses (antra of Highmore) are located in the cheekbones, under the eyes. The ethmoid sinuses, also called ethmoid labyrinth are located between the eyes and the nose. The sphenoid sinuses are located in the center of the skull, behind the nose and the eyes.
The paranasal sinuses are hollow, air-filled cavities that are lined by a mucous membrane. The ethmoid and maxillary sinuses are present at birth. The frontal sinus develops about the second year of life and the sphenoid about the third year. Sometimes, the frontal sinuses do not develop.
The sinuses have small orifices (ostia) that open into recesses in the nasal cavities called meati. The meati are covered by the turbinates (also called conchae) which consist of bony shelves surrounded by erectile soft tissue. There are three turbinates and three corresponding meati in each nasal cavity (superior, middle and inferior).
Endoscopic image of the left middle meatus showing the left maxillary sinus ostium and the middle turbinate.