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.