Ethmoidal labyrinth

The ethmoidal labyrinths are the two box-shaped structures on the sides of the ethmoid bone, flanking the perpendicular plate of ethmoid bone. This configuration resembles the letter 'M', with the perpendicular plate forming the middle limb and the ethmoidal labyrinths as the side limbs. These labyrinths lie between the nasal cavity and the eye sockets (orbits), each comprising two thin sheets of bones that create external and internal walls. These walls encase the ethmoidal air cells, which are categorized into anterior, middle and posterior groups and are enclosed by surrounding bones in a complete skull.

The outer side of each labyrinth (i.e. the orbital plate), which helps form the medial wall of the eye socket, is smooth. On the other hand, the inner side, facing the nasal cavity, forms the lateral nasal wall and displays several important structures. Notably, the superior and middle conchae are two curved bony plates projecting downwards. Below these conchae are their corresponding channels, the superior and middle meatuses, where mucous from the paranasal air sinuses drains into the nasal cavity. Within the middle meatus, the middle ethmoidal air cells protrude, forming the ethmoidal bulla. A small, thin bone known as the uncinate process extends backward from the bottom of the labyrinth. It runs beneath the bulla, forming a curved channel called the semilunar hiatus. The hiatus extends forward and upward, creating a passage called the infundibulum. This passage then continues upward into the ethmoidal labyrinth, forming the frontonasal duct. The frontonasal duct is responsible for draining the frontal sinus and anterior ethmoidal air cells.

In a skull where all bones are connected, the ethmoid fits into the frontal bone's ethmoidal notch. This action covers the top of each labyrinth and forms the roof over the ethmoidal air cells. Here, the anterior and posterior ethmoidal grooves become canals transmitting nerves. At the front, the labyrinths join the frontal process of the maxilla and the lacrimal bone, while at the bottom, they connect with the upper medial margin of the maxilla. The posterior air cells are capped by the sphenoidal conchae and the orbital process of the palatine bone.