Ethmoid Bone

00:01 The ethmoid bone is a single cuboidal shaped bone that's located between the two orbital cavities.

00:09 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.

00:28 So let's first list and visualize these parts and in the subsequent slides, we will take a more detailed view of each part.

00:37 The ethmoid bone has a medially located median plate, which is divided by the cribriform plate into two parts.

00:46 One, the most superiorly located part is the crista galli and two, the inferiorly located perpendicular plate.

00:54 Let's review that one more time because we have now mentioned 3 out of the 4 parts of the ethmoid bone.

01:01 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.

01:13 And the last, the fourth part of the ethmoid bone would be the paired ethmoidal labyrinths.

01:20 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.

01:30 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.

01:43 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.

01:54 However, the cribriform plate through its numerous foramina permits the passage of the axons of the olfactory nerve.

02:02 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.

02:18 That's all on the cribriform plate of the ethmoid.

02:20 Now let's discuss the median plate and its two components: the crista galli and the perpendicular plate.

02:26 The crista galli is a triangular superior part of the median plate of the ethmoid.

02:32 It projects upwards from the cribriform plate.

02:35 And in English it literally translates into the "roosters comb" as it resembles the crest on a roosters head.

02:44 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.

02:56 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.

03:10 This plate articulates with several bones in order to form the upper part of the nasal septum.

03:17 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.

03:30 And lastly, the vomer as well as the nasal septal cartilages that lie inferior.

03:36 And now for the last remaining portion in our discussion of the ethmoid bone, the ethmoidal labyrinths.

03:43 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.

03:57 Here we will go over its major anatomical components without getting too bogged down by minute details.

04:06 The ethmoidal labyrinths are airspaces or cells, which are enclosed by two thin plates of bone.

04:13 These air cells are broken down into three groups based on their location.

04:19 We have an anterior group, a middle group, and then we have a posterior group of air cells.

04:26 These air cells are closed off by adjoining bones, but there are certain locations where these groups drain into the nasal meatuses.

04:36 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.

04:47 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.

05:00 The medial plate on the other hand forms the lateral nasal wall.

05:05 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.

05:19 While we are discussing the nasal cavity, one other thing I would like to touch upon is the inferior nasal concha.

05:27 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.

05:36 So what about the inferior nasal concha? Well, this is a separate spongy bone which is attached to the lateral wall of the nose.

05:44 So remember, the inferior nasal concha is a separate bone of its own.

05:49 That is all in the ethmoid bone