What is the name of a cavity within a bone that is filled with air and lined with a mucus membrane?

• A. Sinus
• B. Fontanel
• C. Notch
• D. Meatus

Answer: (A)

A cavity inside a bone that is filled with air and lined by a mucous membrane is called a sinus. These spaces are typically found in the skull bones—frontal, maxillary, ethmoid, and sphenoid—and are connected to the nasal cavity. The mucous membrane lining helps humidify and filter air, while the air-filled nature lightens the skull, aids in warming inhaled air, and contributes to voice resonance. The other terms describe different structures: a fontanel is a soft spot where skull bones haven’t fused yet, a notch is an indentation along a bone edge, and a meatus is a canal or passageway within a bone. The defining feature here is an air-filled cavity within a bone lined with mucous membrane, i.e., a sinus.