Air cells in medical terms are air-filled cavities in bones. In human beings they are present in the bones of the skull and connect to the nasal cavity and middle ear.
Examples for air cells are the Cellulae mastoideae (mastoid cells) of the Processus mastoideus of the temporal bone which are connected to the middle ear. Additional air cells are:
The function of air cells is not explained yet. Probable one of their functions is to reduce the weight, increasing the surface of the mucous membrane and infection defense