The Sinus Cavities LEGEND: F - Frontal sinuses, E - Ethmoid sinuses, M - Maxillary sinuses, O - Maxillary sinus ostium, SS - Sphenoid sinus ST- Superior turbinate, T - Middle turbinate, IT- Inferior turbinate, SM- Superior meatus, MM- Middle meatus, SR - Sphenoethmoidal recess, S- Septum, ET - Eustachian tube orifice, A - Adenoids The maxillary sinuses (M) drain through the maxillary sinus ostia (O) into the middle meatus (MM). It should be noted that in this graphic diagram, the opening at O appears to be extremely large. In actuality, it is the size of a pin head and actually follows a rather circuitous route as you will see on the CT scans which follow. The ethmoid sinuses (E) drain into both the middle meatus as well as into the superior meatus (SM). The middle meatus (MM) is bounded by the middle turbinate (T) and the inferior turbinate (IT). (There is also a superior turbinate (ST), but that is relatively unimportant.)