The paranasal sinuses are a collection of irregular air-filled bony cavities that are found within the bones of the skull. They are positioned around the nasal cavity. Each person usually has four sets of paired sinuses that are named after the bone in which they are located:

Maxillary sinus - found within the body of the maxilla; this is the largest paranasal sinus;
Ethmoidal air cells - located in the ethmoidal labyrinth;
Frontal sinus - positioned in the nasal part of the frontal bone;
Sphenoid sinus - situated in the body of the sphenoid.

From the most anterior to the most posterior, the paranasal sinuses are positioned as follows: maxillary sinus and frontal sinus, ethmoidal air cells, and sphenoidal sinus. All paranasal sinuses open and drain into the nasal cavity, and they are lined with the same mucous membrane (covered by the pseudostratified ciliated columnar epithelium). Compared to the nasal cavity, the paranasal sinuses contain fewer goblet cells and glands, as well as they do not have venous plexuses.

The ethmoidal air cells and maxillary sinuses are usually present at birth (but they are very small). The frontal sinuses start to develop between the ages of 4 and 7. Sometimes, they never develop. The sphenoidal sinuses usually become visible at about (2) 3 to 7 years of age.

The functions of the paranasal sinuses are still being explored and are not yet completely understood. However, it is known that they humidify, transmit and warm the inhaled air and significantly decrease the weight of the head, which impacts the resonance of the voice. They also provide immunological defense and facilitate the immune response of the respiratory system.

Ethmoidal air cells

The ethmoidal air cells (also called ethmoidal sinuses) are small air-filled spaces positioned in the ethmoid. They are situated between the bony orbits on either side of the nasal septum.

At birth, a baby has 3 to 4 ethmoidal air cells that later develop into 5 to 15 paired cells. The total volume of these cells is 0.068 to 0.10 oz (2 to 3 ml).

The ethmoidal air cells are divided into three groups:

Anterior ethmoidal cells - the most anterior located cells; they are covered by the lacrimal bone; these cells are drained to the middle nasal meatus via the ethmoidal infundibulum;
Middle ethmoidal cells - the middle cells; they are covered by the orbital plate of the ethmoid; they are drained into the middle nasal meatus;
Posterior ethmoidal cells - the most posterior located spaces; they are also covered by the orbital plate of the ethmoid; these cells are drained into the superior nasal meatus.

The largest anterior ethmoidal air cell is called the bony ethmoidal bulla.

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Paranasal sinuses and ethmoidal air cells