The ethmoid bone is a single cuboidal shaped bone
that's located between the two orbital cavities.
The ethmoid bone has four parts,
which contribute in various ways
and in various degrees to the formation
of the medial wall of the ocular orbit,
the nasal septum, as well as the roof
and the lateral walls of the nasal cavity.
So let's first list and visualize these
parts and in the subsequent slides,
we will take a more detailed view of each part.
The ethmoid bone has a medially located median plate,
which is divided by the
cribriform plate into two parts.
One, the most superiorly
located part is the crista galli
and two, the inferiorly located perpendicular plate.
Let's review that one more time
because we have now mentioned
3 out of the 4 parts of the ethmoid bone.
The first part again is the cribriform plate,
and the second part is the crista galli,
and the third part, the perpendicular
plate which along with the crista galli
forms the median plate of the ethmoid.
And the last, the fourth part of the ethmoid
bone would be the paired ethmoidal labyrinths.
Now, as promised, let's take a closer
look at these four parts of the ethmoid bone:
the cribriform plate, the crista galli, the
perpendicular plate and the ethmoidal labyrinths.
The cribriform plate is a thin sieve-like
shelf that fills the ethmoidal notch,
which, if you remember was the recess which
separated the orbital plates from each other.
The cribriform plate also contributes to
the formation of the roof of the nasal cavity
through which it separates the nasal
cavity from the anterior cranial fossa.
However, the cribriform plate
through its numerous foramina
permits the passage of the
axons of the olfactory nerve.
Here we see the passage of the axons of the
olfactory nerve into the anterior cranial cavity
in order to synapse with the
olfactory bulb so that sensation of smell
can be conveyed from the nasal cavity.
That's all on the cribriform plate of the ethmoid.
Now let's discuss the median
plate and its two components:
the crista galli and the perpendicular plate.
The crista galli is a triangular superior
part of the median plate of the ethmoid.
It projects upwards from the cribriform plate.
And in English it literally
translates into the "roosters comb"
as it resembles the crest on a roosters head.
This bony structure is important because its
posterior border gives attachment to the falx cerebri
and invagination of the dura mater which
separates the two cerebral hemispheres.
The perpendicular plate of the
ethmoid is a quadrangular thin flat bone
which represents the inferior part
of the medium plate of the ethmoid
and it projects downward from the cribriform plate.
This plate articulates with several bones in
order to form the upper part of the nasal septum.
These articulations include the
nasal spine of the frontal bone,
and the crest of the nasal bones anteriorly
it includes the sphenoidal
rostrum or body that lies posterior.
And lastly, the vomer as well as the
nasal septal cartilages that lie inferior.
And now for the last remaining portion
in our discussion of the ethmoid bone,
the ethmoidal labyrinths.
The ethmoidal labyrinths are truly
as the name suggests, labyrinths
and they can be quite difficult
to imagine and visualize
and their detailed anatomy is only of
true interest to otorhinolaryngologist.
Here we will go over its major anatomical components
without getting too bogged down by minute details.
The ethmoidal labyrinths are airspaces or
cells, which are enclosed by two thin plates of bone.
These air cells are broken down into
three groups based on their location.
We have an anterior group, a middle group,
and then we have a posterior group of air cells.
These air cells are closed off by adjoining
bones, but there are certain locations
where these groups drain into the nasal meatuses.
For example, the posterior air cells
open up into the superior nasal meatus
whereas the middle ethmoidal air cells
open up into the middle nasal meatus.
The lateral plate of the ethmoidal
labyrinths, which encloses these air cells
contributes to the formation
of the medial orbital wall
and as such, it is often termed
the lateral orbital plate.
The medial plate on the other
hand forms the lateral nasal wall.
And also from this plate we have
small curved shelf like lamellae
that originate to form the middle, superior
and uncommonly the supreme nasal conchae.
While we are discussing the nasal cavity,
one other thing I would like to touch upon
is the inferior nasal concha.
As we just mentioned, the medial plate of the
ethmoid bone gives rise to the middle, superior
and in certain cases, as supreme nasal conchae.
So what about the inferior nasal concha?
Well, this is a separate spongy bone which
is attached to the lateral wall of the nose.
So remember, the inferior nasal
concha is a separate bone of its own.
That is all in the ethmoid bone