The frontal sinus is formed from the expansion of an anterior ethmoidal cell into the frontal bone or as a direct extension of the infundibulum.
Usually there are two sinuses, but it is important to remember that one or both may be absent, and that there may be as many as four or five distinct sinuses, communicating with the nasal cavity by their own ostia.
One sinus may be underdeveloped, and the sinus of the opposite side may have expanded beyond the median line and formed part of the superior and anterior wall of the smaller one, and also an anterior ethmoidal cell may have a large orbital extension and its anterior wall bulge into the posterior part of the frontal sinus, forming the so-called bulla frontalis; and this ethmoidal cell may be the seat of disease which will simulate frontal sinus disease, while the frontal sinus is normal.
LEE M. HURD. INTRANASAL SURGERY FOR RELIEF OF CHRONIC FRONTAL SINUSITIS. JAMA. 1916;LXVII(25):1816–1817. doi:10.1001/jama.1916.02590250018004