The accessory sinuses or air cells of the nose are the frontal, ethmoidal, sphenoidal, and maxillary; they vary in size and form in different individuals, and are lined by ciliated mucous membrane directly continuous with that of the nasal cavities.
The Frontal Sinuses (sinus frontales), situated behind the superciliary arches, are rarely symmetrical, and the septum between them frequently deviates to one or other side of the middle line. Their average measurements are as follows: height, 3 cm.; breadth, 2.5 cm.; depth from before backward, 2.5 cm. Each opens into the anterior part of the corresponding middle meatus of the nose through the frontonasal duct which traverses the anterior part of the labyrinth of the ethmoid. Absent at birth, they are generally fairly well developed between the seventh and eighth years, but only reach their full size after puberty.
The Ethmoidal Air Cells (cellulæ ethmoidales) consist of numerous thin-walled cavities situated in the ethmoidal labyrinth and completed by the frontal, maxilla, lacrimal, sphenoidal, and palatine. They lie between the upper parts of the nasal cavities and the orbits, and are separated from these cavities by thin bony laminæ. On either side they are arranged in three groups, anterior, middle, and posterior. The anterior and middle groups open into the middle meatus of the nose, the former by way of the infundibulum, the latter on or above the bulla ethmoidalis. The posterior cells open into the superior meatus under cover of the superior nasal concha; sometimes one or more opens into the sphenoidal sinus. The ethmoidal cells begin to develop during fetal life.
The Sphenoidal Sinuses (sinus sphenoidales) contained within the body of the sphenoid vary in size and shape; owing to the lateral displacement of the intervening