Ethmoid bone

The ethmoid bone (os ethmoidale) is an unpaired bone of the neurocranium, although most of it makes up the viscerocranium. The ethmoid bone is located centrally between the bones of the visceral cranium and comes in contact with most of them to form the nasal and orbital cavities. The bony plates of the ethmoid bone are arranged in the form of the letter “T”, in which the vertical line is formed by the perpendicular plate and the horizontal one is formed by the cribriform plate. From the cribriform plate and on each side of the horizontal plate hang the ethmoidal labyrinths.

The ethmoid bone consists of four parts:

Cribriform plate (lamina cribrosa) is a rectangular plate that fits into the ethmoid notch of the frontal  bone. It is perforated by small openings like a sieve (Grethmos = sieve, eidos = form), which are called the cribriform foramina (foramina cribrosa). These perforations transmit the olfactory nerves (CN I).

Crista galli (crista galli) with the ala of crista galli (ala cristae galli) projecting upward from the midline of the cribriform plate for the attachment of the dura mater.

Perpendicular plate (lamina perpendicularis)  is part of the nasal septum.

The ethmoidal labyrinth (labyrinthus ethmoidalis) makes up a paired complex of bony air ethmoidal cells (cellulae ethmoidales). On the inferior surface of the ethmoidal labyrinth are present the largest air cells called ethmoidal bulla (bulla ethmoidalis). They are situated under the middle nasal concha. Air cells communicate  with the middle nasal meatus through a canal called the ethmoidal infundibulum (infundibulum ethmoidale). The ethmoidal infundibulum is a space between the uncinate process (anteroinferiorly) and the ethmoidal bulla (posterosuperiorly).

The uncinate process (procesus uncinatus) is a bony plate covered by mucosa. The posterior end of it connects with the perpendicular plate of the palatine bone and ethmoidal processes of the inferior nasal concha (inferiorly). Its posterior mar gin (superior part) is located anteriorly to the ethmoidal bulla. Between the posterior margin and ethmoidal bulla there is a space  referred to the semilunar hiatus, which an teriorly is referred to (only) the ethmoidal infundibulum.

On the medial surface of the labyrinth there are the superior and middle nasal conchae (concha nasalis superior et media), although sometimes there is a third, the supreme nasal concha (concha nasalis suprema). The lateral surface of the labyrinth is covered laterally by a thin orbital plate (lamina orbitalis), which forms the medial wall of the orbit.

The orbital plate articulates with the:

  1. Orbital part of the frontal bone.
  2. Lacrimal bone at the anterior border.
  3. Orbital process of the palatine bone at the posterior border.
  4. Maxilla at the inferior border.