The ethmoid bulla (or ethmoidal bulla) is a rounded elevation upon the lateral wall of the middle nasal meatus[1][2]: 377 (nasal cavity inferior to the middle nasal concha) produced by one or more of the underlying middle ethmoidal air cells (which open into the nasal cavity upon or superior to the ethmoidal bulla[2]: 374 [3]).
[2]: 377 It varies significantly based on the size of the underlying air cells.
[3] The size of the bulla varies with that of its contained cells.
The bulla may be a pneumatised cell or a bony prominence found in middle meatus.
[citation needed] The hiatus semilunaris is situated (sources differ) inferior[5][6][1]/anterior[2]: 374 to the ethmoid bulla.