The ethmoidal cells are formed by a variable number of individual air chambers, which are divided into anterior, middle, and posterior ethmoidal cells based on the location of their apertures on the lateral wall of the nasal cavity:
The anterior ethmoidal cells open into the ethmoidal infundibulum or the frontonasal duct.
The middle ethmoidal cells open onto the ethmoidal bulla, or onto the lateral wall just above this structure.