**4. Frontal sinus physiology**

The mucosa of the frontal sinus consists of pseudostratified ciliated epithelium, mucus producing goblet cells, a thin basement membrane, and a thin lamina propria that contains seromucous glands. It covers the entire surface of the sinus and ranges in thickness from 0.07 to 2.0 mm. When the mucosa is healthy, a blanket of mucin overlies the epithelium. The cilia beat at 250 cycles/min. The mucin blanket flows in a spiral fashion in a medial-to-lateral direction; the flow is slowest at the roof and fastest at the NFD. The mucin empties at the NFD at a daily rate of 5.0 g/cm2. [14, 19, 20,]

The frontal sinus is unique in that it is the only sinus that has a recirculation phenomenon. The mucus travels along the lateral side of the sinus and turns medially over the sinus floor and down the lateral frontal recess wall. Of the secretion, 60% is directed back into the sinus cavity as it reaches the frontal recess. [13] Clinically significant anatomical structures of the mucosa of the frontal sinus are the foramina of Breschet, first described over 60 years ago. These foramina are sites of venous drainage of the mucosa and can serve as the route of intracranial spread of infection. The mucosa is found deeply penetrating these foramina. If mucosa is not completely removed microscopically from these foramina in obliteration or cranialization procedures, there is a high risk of mucocele formation. [13]
