Sinuses are mucosa-lined air spaces within the bones of the face and skull. The frontal sinuses are situated behind the superciliary arches. The frontal sinuses are absent at birth, but are generally fairly well developed between the seventh and eighth years, only reaching their full size after puberty. The frontal bone is membranous at birth and there is rarely more than a recess until the bone tissue starts to ossify about age two. Consequently this structure does not show on radiographs before that time. Frontal sinuses are rarely symmetrical and the septum between them frequently deviates to one or other side of the middle line. Sinus development begins in the womb, but only the maxillary and ethmoid sinuses are present at birth. The frontal sinuses are absent at birth, they become well developed between the seventh and eighth years, and reach their full size after puberty. Through its copious mucus production, the sinus is an essential part of the immune defense/air filtration carried out by the nose. Nasal and sinal mucosae are ciliated and move mucus to the choanae and finally to the stomach. The thick upper layers of nasal mucus trap bacteria and small particles in tissue abundantly provided with immune cells, antibodies, and antibacterial proteins. The layers beneath are thinner and provide a substrate in which the cilia are able to beat and move the upper layer with its debris through the ostia toward the choanae.