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Ethmoid bone lies between two orbital cavities, cuboidal in shape. It takes part in the formation of orbit, anterior cranial fossa, roof, lateral wall and septum of nose.
Parts:
It has three parts:
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1. Cribriform plate,
2. Perpendicular plate and
3. Labyrinthine part.
At the time of birth bone has three pieces- median plate, right and left labyrinths.
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1. Cribriform Plate:
It is sieve like, horizontal, fragile plate – forms part of floor of anterior cranial fossa and roof of nose.
It has two surfaces – superior and inferior surface and four borders – anterior, posterior, right and left lateral borders.
Superior Surface:
Shows an upward projection called crista galli, situated in the midline, gives attachment to falx cerebri. Anterior to crista galli lies a foramen called foramen caecum and lateral to it sieve like cribriform plate lies. Olfactory nerve passes through the openings.
Lateral to crista galli- A slit is present lodging duramater, lateral to slit anterior and posterior ethmoidal foramen is situated which transmits anterior and posterior ethmoidal nerve and vessels.
Inferior Surface:
Forms roof of the nose. From the middle of this surface perpendicular plate projects downwards and forms bony part of nasal septum.
2. Perpendicular Plate:
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It is quadrangular in shape. Anterior border articulates with the frontal and nasal bones. Posterior border articulate with sphenoid bone above and vomer bone below. Upper border articulates with cribriform plate and lower border is attached to septal cartilage.
3. Labyrinthine Part:
It contains ethmoidal air sinuses. It has superior, inferior, posterior, medial and lateral surfaces.
Superior Surface:
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Contains air cells. These air cells are complete by articulating with orbital plate of frontal bone.
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Inferior Surface:
It is articulating anteriorly with the body of maxilla and orbital process of palatine bone posteriorly.
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Anterior Surface:
Articulates with frontal process of maxilla and lacrimal bone.
Posterior Surface:
Large air cells present. They are closed by sphenoidal concha and orbital process of the palatine bone. The labyrinthine part forms superior and middle concha of noses.
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Medial Surface:
Forms lateral wall of the nose.
Lateral Surface:
It is very thin, covers the middle and posterior ethmoidal air cells and takes part in formation of medial wall of the orbit.
Ethmoidal Air Cells:
These are anterior, middle and posterior. Anterior cells opens into anterior part of hiatus semilunasis present in the middle meatus of nose. Middle cells form bulla ethmoidalis and opens on its surface. The posterior ethmoidal cells drain into superior meatus present in the lateral wall of nose.
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From the anterior end of the labyrinth – a process is projecting downwards – called uncinate process. Deep to it hiatus semi lunaris is situated.
Ossification is by three primary centres. Two centres appear at 16-28 weeks of intra uterine life for labyrinth and one for perpendicular plate appears near crista galli around first year.
During second year ossification completes and labyrinth unites with perpendicular plate of ethmoid bone.
