The ethmoid bone is cuboidal in overall shape (Fig. 8.233A) and is composed of two rectangular box- shaped ethmoidal labyrinths, one on each side, united superiorly across the midline by a perforated sheet of bone (the cribriform plate). A second sheet of bone (the perpendicular plate) descends vertically in the median sagittal plane from the cribriform plate to form part of the nasal septum.