Human Anatomy, Part 1 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2002

Special Part
Axial Skeleton, sceleton axiale - Bones of the Neurocranium
Ethmoid bone

The ethmoid bone, os ethmoidale (Fig. 43) is an unpaired bone that develops in connection with the nasal capsule on a cartilaginous basis. Although systematically classified alongside the cranial bones, the greater part of it contributes to The formation of the facial Skeleton. It is located in the upper PARTS OF THE Nasal cavity (within the ethmoidal notch, incisura ethmoidalis, of the Frontal bone) and the anterior parts of the cranial base. It consists of a horizontal cribriform plate, lamina cribrosa, and a perpendicular plate, lamina perpendicularis, as well as the ethmoidal labyrinths, labyrinthus ethmoidalis, situated on both sides of the latter.

The horizontal cribriform plate contributes to the Formation of the fronto-ethmoidal suture, sutura fronto-ethmoidalis. In its upper part, the perpendicular plate forms the crista galli. The anteroinferior margin of the crest is flanked on each side by the inconstant wings of the crista galli, ala cristae galli, or alar processes. These processes bound the foramen cecum of the frontal bone. The labyrinth of the ethmoid bone, labyrinthus ethmoidalis, is a paired Structure located on either side of the perpendicular plate. Each labyrinth is composed of ethmoidal Cells, cellulae ethmoidales, which are lined with mucous membrane and communicate with the nasal cavity and conchae. The spread of infection into the ethmoidal cells leads to inflammation of their mucous membrane, known as ethmoiditis, etmoiditis. Typically, the ethmoid bone contains 8 to 10 ethmoidal cells, though cases ranging from 5 to 14 have been reported. The ethmoidal cells are arranged in 3 or 4 rows corresponding to the number of ethmoidal conchae (V. P. Vorobyov, 1932). The cells are divided into anterior, cellulae ethmoidales anteriores, middle, cellulae ethmoidales mediae, which open into the middle nasal meatus, and posterior, cellulae ethmoidales posteriores, which open into the superior nasal meatus. The lateral wall of the labyrinth is formed by the orbital plate, lamina orbitalis, which participates in the formation of the fronto-ethmoidal, ethmoidomaxillary, palato-ethmoidal, lacrimal-ethmoidal, and spheno-ethmoidal sutures.

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Fig. 43. Ethmoid bone, os ethmoidale: A - superior view; B - inferior view

Along the upper margin of the labyrinth run the anterior and posterior ethmoidal grooves, which form the ethmoidal foramina, foramina ethmoidalia. On the medial wall of the labyrinth lie the superior and middle nasal conchae, concha nasalis superior et media; occasionally, a third, supreme nasal concha, concha nasalis suprema, is also present. The superior nasal meatus, meatus nasi superior, is formed between the superior nasal concha above and the middle nasal concha below. Extending from the anteroinferior surface of the labyrinth is the uncinate process, processus uncinatus, behind which lies the ethmoidal bulla, bulla ethmoidalis (a bulge formed by one of the ethmoidal labyrinth cells). Between the uncinate process and the ethmoidal bulla lies the ethmoidal infundibulum, infundibulum ethmoidale, a cleft-like channel that connects the middle nasal meatus with the frontal sinus at its anterior end.

Ossification. The ethmoidal labyrinth develops endochondrally. The first ossification center appears in the middle nasal concha at the 16th week (M. Augier, 1931), and in the superior nasal concha at the 20th week. By the end of the first year of life, ossification centers appear in the crista galli and the perpendicular plate, with Ossification of the latter being completed by the 6th to 8th years of life. The final formation of the air cells occurs at 12 to 14 years of age (A. Andronescu, 1970).