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The ethmoid bone (/ ˈ ɛ θ m ɔɪ d /; [1] [2] from Ancient Greek: ἡθμός, romanized: hēthmós, lit. 'sieve') is an unpaired bone in the skull that separates the nasal cavity from the brain. It is located at the roof of the nose, between the two orbits. The cubical bone is lightweight due to a spongy construction.
In the ethmoid bone, a sickle shaped projection, the uncinate process, projects posteroinferiorly from the ethmoid labyrinth. Between the posterior edge of this process and the anterior surface of the ethmoid bulla, there is a two-dimensional space, resembling a crescent shape.
In mammalian anatomy, the cribriform plate (Latin for lit. sieve-shaped), horizontal lamina or lamina cribrosa is part of the ethmoid bone.It is received into the ethmoidal notch of the frontal bone and roofs in the nasal cavities.
The ethmoidal labyrinth or lateral mass of the ethmoid bone consists of a number of thin-walled cellular cavities, the ethmoid air cells, arranged in three groups, anterior, middle, and posterior, and interposed between two vertical plates of bone; the lateral plate forms part of the orbit, the medial plate forms part of the nasal cavity.
The frontonasal duct may or may not drain into the ethmoidal infundibulum - this is determined by the place of attachment of the uncinate process of the ethmoid bone: if the uncinate process is attached to the lateral nasal wall, the frontonasal duct will open directly into the middle nasal meatus; if otherwise, it will drain into the infundibulum.
The ethmoidal notch separates the two orbital plates; it is quadrilateral, and filled, in the articulated skull, by the cribriform plate of the ethmoid.. The margins of the notch present several half-cells which, when united with corresponding half-cells on the upper surface of the ethmoid, complete the ethmoidal sinuses.
The olfactory foramina, also known as the cribriform foramina (cribr- is "a sieve" in Greek), is the grouping of holes located on the cribriform plate.The cribriform plate forms the roof of the nasal cavity, and the olfactory foramina are in the two depressions lateral to the median blade of the cribriform plate called the crista galli.
Endochondral ossification is responsible for development of most bones including long and short bones, [4] the bones of the axial (ribs and vertebrae) and the appendicular skeleton (e.g. upper and lower limbs), [5] the bones of the skull base (including the ethmoid and sphenoid bones) [6] and the medial end of the clavicle. [7]