The Sinus Cavities The three overlapping flaps of tissue, called turbinates (inferior - IT, middle - T, and superior - ST ) protect the openings of the sinuses, and allow humidification, filtration and warming of air. The frontal (F) sinus is seen in this view, which is not usually involved to any great extent in sinusitis. The sphenoid sinus (SS) is also seen in this view, and is sometimes involved in sinusitis. The sphenoid sinus drains into the sphenoethmoidal recess (SR)