Cilia Motion... The mucosal lining of the sinus is covered with hair cells that actively pump fluid towards the exit. The mucous that is produced by the lining of the sinus flows along in a specific path that is determined by the direction that the hair cells beat. In health, a thin blanket of mucous is constantly flowing along. The surface layer is in a gel type phase, the deeper layer is a less viscous phase, called sol. 1999 Peter Casano, M.D.