The Ethmoid Bone
The ethmoid bone is a singular porous bone that makes up the middle area of the viscerocranium and forms the midfacial region of the skull. It contributes to the moulding of the orbit, nasal cavity, nasal septum and the floor of the anterior cranial fossa.
The bone consists of a perpendicular plate and two ethmoidal labyrinths parts which are all superiorly attached to the cribriform plate. A smaller orbital part extends towards the orbit.
The ethmoid labyrinths lie on both lateral sides and contain numerous little cavities with ethmoidal cells which are referred to as the ethmoidal sinus. The labyrinths form two of the biggest structures in the nasal cavity: the superior and middle nasal concha. The hiatus semilunaris separates the ethmoid bulla and the uncinate process. It constitutes the connection between the frontal and maxillary sinuses to the anterior ethmoidal cells.
The perpendicular plate is a thin lamina which runs horizontally from the cribriform plate. Inferiorly it attaches to the septal cartilage of the nose and hereby forms part of the nasal septum.
The cribriform plate (Latin “cribriform” = perforated) lies within the ethmoidal notch of the frontal bone and forms the roof of the nasal cavity. As the name suggests it comprises numerous openings through which the olfactory fibers from the nasal cavity pass through to the anterior cranial fossa. The falx cerebri is attached to the crista galli (Latin “crista galli” = crest of the cock), a small vertical protrusion on top of the plate. The olfactory bulbs lie on two grooves lateral to the crista galli.
Because of its central location within the skull the ethmoid bone comes in contact with 15 other skull bones. The most important borders are:
- anterior with the frontal bone
- posterior with the sphenoid bone
- inferior with the vomer and inferior nasal concha
The ethmoid bone ossifies completely by endochondral ossification. In newborns the labyrinths are relatively small and both the perpendicular and cribriform plates consists mainly out of cartilage. The latter start to ossify by the age of one and fuse with the labyrinths by the age of two to form a single ethmoid bone.
- a perpendicular plate - a thin lamina which runs horizontally from the cribriform plate and forms part of the nasal septum.
- two ethmoidal labyrinths - parts which are all superiorly attached to the cribriform plate. They contain numerous little cavities with ethmoidal cells which are referred to as the ethmoidal sinus
- cribriform plate comprises numerous openings through which the olfactory fibers from the nasal cavity pass through to the anterior cranial fossa.
The ethmoid bone also has several borders, the most important being mentioned below:
- Frontal lobe - anteriorly
- Sphenoid bone - posteriorly
- Vomer + Inferion nasal chonca - inferiorly