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air cavities in the cranial bones that communicate with the nasal cavity by means of narrow canals or fissures.
The maxillary sinus, or antrum of Highmore, is located in the maxillary bone; the opening connecting it to the nasal cavity is situated in the meatus nasi medius. The frontal sinus is located between the membranes of the orbital part and the squama frontalis; it communicates with the nasal cavity by way of the nasofrontal duct, which opens into the anterior part of the meatus nasi medius. The ethmoidal labyrinth consists of two to five or more air cells of varying size and shape. The anterior cells open into the meatus nasi medius, and the posterior cells into the meatus nasi superior. The sphenoidal, or wedge-shaped, sinus is located in the body of the sphenoid bone immediately behind the ethmoidal labyrinth; the anterior wall of each half of the sinus has openings that connect it to the nasal cavity.
The mucous membrane lining of the paranasal sinuses is similar in structure to that of the nasal cavity but is considerably thinner and relatively lacking in vessels and glands. The most common disturbances of the paranasal sinuses are caused by inflammations of their mucosa, or sinusitis, and by traumas.