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Figure 52-1. Scheme of the ethmoid bone viewed from behind. The two ethmoidal labyrinths are united by the cribriform plate. The perpendicular plate, which forms the upper part of the nasal septum, is set at a right angle to the horizontally placed cribriform plate. The lateral surface of each labyrinth forms a part of the medial wall of the orbit and is termed the orbital plate of the ethmoid bone. F indicates the portion of the labyrinth completed by the frontal bone; 5, the portion completed by the sphenoid bone. The ethmoidal labyrinth contains ethmoidal air cells known collectively as the ethmoidal sinus. (After Grant.) Click for high resolution image.