The ethmoid bone is cuboidal in overall shape and contains the ethmoidal cells (ethmoidal sinuses). It is composed ofTwo rectangular box-shaped ethmoidal labyrinths, one on each side. They are united superiorly across the midline by a perforated sheet of bone - the cribriform plate. The perpendicular plate - A second sheet of bone descends vertically in the median sagittal plane from the cribriform plate to form part of the nasal septum.Ethmoidal labyrinth: Each ethmoidal labyrinth is composed of two delicate sheets of bone, which sandwich between them the ethmoidal cells. ■ The lateral sheet of bone (the orbital plate) is flat and forms part of the medial wall of the orbit. ■ The medial sheet of bone forms the upper part of the lateral wall of the nasal cavity and is characterized by two processes and a swelling—The two processes are curved shelves of bone (the superior and middle conchae), which project across the nasal cavity and curve downward ending in free medial margins, while inferior to the origin of the middle concha, the middle ethmoidal cells form a prominent bulge (the ethmoidal bulla), on the medial wall of the labyrinth. Agger nasi - runs forward and downwards from upper end of anterior border of middle concha.TO NOTE: Inferior concha/ inferior nasal turbinate is an independant bone, not a projection of ethmoidal labyrinth. Ethmoidal infundibulum:- Extending anterosuperiorly from just under the bulla is a groove (the ethmoidal infundibulum), which continues upward, and narrows to form a channel that penetrates the ethmoidal labyrinth and opens into the frontal sinus. This channel is for the frontonasal duct, which drains the frontal sinus. Uncinate process:- A delicate irregularly shaped projection (the uncinate process) on the anterior aspect of the inferior surface of the ethmoidal labyrinth. Cribriform plate:The cribriform plate is at the apex of the nasal cavities and fills the ethmoidal notch in the frontal bone and separates the nasal cavities below from the cranial cavity above. Small perforations in the bone allow the fibers of the olfactory nerve to pass between the two regions. A large triangular process (the crista galli) at the midline on the superior surface of the cribriform plate anchors a fold (falx cerebri) of dura mater in the cranial cavity.Perpendicular plate of ethmoid:The perpendicular plate of the ethmoid bone is quadrangular in shape, descends in the midline from the cribriform plate, and forms the upper part of the median nasal septum.