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Ethmoid bone

Structure and landmarks of the ethmoid bone.

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Hello everyone! This is Joao from Kenhub, and welcome to this tutorial where today we're going to be talking about the ethmoid bone. The very first thing we see here is this image where we're looking at the skull from the side as if it were cut in half right down the middle. We are looking at the inside of the right half of the skull. From this view, we can see a distinction between this structure that we're now highlighting which is known as the viscerocranium and this other structure right here which is known as the neurocranium.

Now, you might be asking, what is viscerocranium? Well, I talked about it in different tutorials here. The viscerocranium is a collective term for the bones of your face essentially. Now, the neurocranium, on the other hand, are the bones that surround the brain including the ethmoid bone. The ethmoid bone is one of the most complicated bones of the skull – just by looking at it, we can see that. So we're going to look at each of its structures individually throughout this tutorial.

Now, let's have a look at the ethmoid bone on its own and to do so, we're going to look at the posterior view of the skull as you can see now on the screen – the posterior view – so we are looking at it from the back. The bone is now seen from its back. Here we see the two large cube-shaped pieces of bone called then the lateral masses. We'll see later that these masses are mostly hollow.

Now between the two lateral masses, you can see highlighted in green a narrow bony plate projecting inferiorly. This one is called the perpendicular plate of the ethmoid bone and it forms part of your nasal septum that separates the right and left nasal cavities. In patients with cleft palate and lip, the shape of the perpendicular plate is often abnormal and this condition we usually refer it to as a deviated septum.

Now, we're looking at the middle of the skull from the side again. The pink layer that has been added is known as the nasal mucosa. This mucosa lines the nasal cavities which you see highlighted here and also the nasopharynx. And in this image, we can see the medial surface of the lateral masses. The surface is characterized by two curved projections called the nasal conchae. A third independent concha articulates with the inferior border of the ethmoid bone to then create four air channels through the right and left nasal cavities.

And as you can see in these pictures, it is quite easy to name the three nasal conchae as the superior, the middle and the inferior one since they are placed one above the other in the medial surfaces of both lateral masses. The most superior air channel is called the sphenoethmoidal recess which you can see here because it is bordered by both the sphenoid and the ethmoid bones.

In this image, the sphenoethmoidal recess is highlighted in green. The channel directly below the superior concha is the superior nasal meatus – right here. There is another channel between the middle and inferior nasal conchae called the middle nasal meatus – which you can see also here – and, finally, there is an inferior nasal meatus which you see here below the inferior nasal concha.

The nasal conchae have an interesting role in breathing. They increase surface area in your nasal cavities and then rotate air passing through the nasopharynx. Rotation of inhaled air increases contact with the nasal mucosa which is extremely important because it helps to humidify the air and to filter it from the microbes or any other harmful inhaled bodies. Now saturating the inhaled air with water prevents the tissues of your lungs from drying out when you breathe quickly or deeply.

Now, this is the inside of a skull as if we had removed the top of the neurocranium – and that was the part of the skull if you were paying attention that houses the brain. The ethmoid bone is right here on this zoomed-in image. We're looking at it from a superior view and, superiorly, the ethmoid bone is characterized by the presence of this structure that you see highlighted in green which is known as the crista galli. On each side of the crista galli, you can see this structure perforated by foramina named the cribriform plate – and we will talk about it a little bit later.

I will add the image of the brain to the image of the superior view of the skull, and we can now see this structure here which is known as the dura mater. The dura mater is the outermost thick covering over the brain which folds in on itself and some areas creating then separations and regions of your brain. Now, the crista galli is the anterior attachment of the falx cerebri which you see here. It's a dural infolding that separates the right and left cerebral hemispheres.

Now, as we already said, next to the crista galli on either side, there is this small plate of bone peppered with foramina called the cribriform plate. These foramina are vital to olfaction which is the fancy and medical term that we use for the sense of smell.

Now, we're going to take a closer look at the cribriform plate and our first our cranial nerve, the olfactory nerve. In this image, we are looking at the close-up of the ethmoid bone. Here we can see the cribriform plate in green separating then the nasal cavities from the cranial cavity. Now as you inhale air, receptors in your nasal mucosa, the roof of the nasal cavity called then olfactory epithelium are stimulated. The axons of these special sensory neurons project superiorly through the foramina to then reach the olfactory bulbs lying on the upper surface of the cribriform plate.

When viewed in a sagittal section, you can see that the olfactory bulbs kind of look like a toothbrush with its axons looking a little bit like bristles projecting through the cribriform plate. From there, sensory signals are projected through the olfactory tract to be processed in the olfactory cortex of the brain. Quick clinical note here that fractures in the floor of the anterior cranial fossa may involve the cribriform plate. What happens then is this may cause the leakage of the cerebrospinal fluid from the nose, a condition that we call CSF rhinorrhea.

If we look at the ethmoid bone from above now, we can see both lateral masses here, the cribriform plate and the crista galli in the middle. In this image, the most lateral part of the lateral masses is highlighted in green. This bony landmark is called the orbital plate. The orbital plate of the ethmoid bone articulates with the frontal bone and the maxilla to make up the medial wall of the bony orbit or your eye socket.

Now, let's move to the inside of a bony orbit from a lateral view and look at the inside of an eye socket as if the eye were removed so we could see the bones closest to our nose from the side. Here is the bridge of the nose and here is your forehead. You can also see here a small part of your cheekbone, and from this perspective, we can see the orbital plate making up the medial wall of the bony orbit. There are two small foramina between the orbital plate and the frontal bone – which is this bone that we're highlighting – and these are the anterior and the posterior ethmoidal foramina. Now remember that foramina is plural for foramen. These foramina transmit the anterior and posterior ethmoidal vessels and nerves which we just added to the image. The ethmoidal vessels and nerves supply the nasal cavity and the ethmoidal air cells.

Moving inside the body of the ethmoid bone, we can see the ethmoidal air cells which are part of the four paranasal sinuses. Paranasal sinus is an air-filled space within the bones of the skull around your nasal cavity. There are maxillary, sphenoidal, frontal and ethmoidal sinuses but we're going to be covering them in more detail on other tutorials. The air cells of each sinus are made up of multiple of air chambers lined by respiratory mucosa. In the ethmoid bone, these air chambers the ethmoid cells that are colored in green on this image are named by their location relative to the lateral wall of the nasal cavity. Anterior ethmoidal air cells which you see here open into the frontonasal duct, middle ethmoidal cells now seen here open onto the ethmoidal bulla which we will see later on, and posterior ethmoidal air cells open onto the lateral wall of the superior nasal meatus.

The ethmoidal air cells receive innervation from the branches of the ophthalmic and maxillary nerves. If foreign invaders such as a virus or bacteria enter the air cells, a patient may develop sinusitis more commonly called as sinus infection. If you ever had a sinus infection, you probably know that the blockage of mucus in a paranasal sinus causes a really, really bad headache.

The ethmoidal bulla which you see now pointed at which I also mentioned before is a small bulge inferior to the middle nasal concha. The bulla is formed by a swelling of middle ethmoidal air cells into the lateral wall of the nasal cavity, and this is where the middle ethmoidal air cells open into the nasal cavity. This bulge creates a small groove called the semilunar hiatus just inferior to the bulla. The opening to the maxillary sinus is located in the floor of the semilunar hiatus. In relation to the ethmoidal cells and ethmoidal bulla, a small groove extends anteriorly from under the bulla. We can see it now – this is the groove called the ethmoidal infundibulum.

The infundibulum courses superiorly and narrows to form a channel through the ethmoidal air cells that opens into the frontal sinus. This channel is the frontonasal duct and skull fractures, for example, following a car accident, the ethmoidal infundibulum and the frontonasal duct must be really carefully assessed. If fractures in the frontonasal duct are not treated, a patient may develop meningitis or a brain abscess. In some cases, bone grafts are necessary to prevent these complications.

Lastly, let's look at this green area extending posteriorly and inferiorly the lateral masses – this is named the uncinate process. The uncinate process is a small projection of bone that forms a small part of the wall of the maxillary sinus and articulates with the inferior nasal concha. An interesting note about the uncinate process that it has become an important surgical landmark with the advent of endoscopic nasal surgery. Identifying and removing the uncinate process improves access to the ethmoidal bulla, frontal sinus and outflow tract of the maxillary sinus.

And that's it for the ethmoid bone but before we finish, I would like to do a quick summary of what we just talked about. In this video, we looked at the structure of the ethmoid bone. We first took a look at the lateral masses of the ethmoid bone. These masses are mostly hollow. Their perpendicular plates form part of the nasal septum. We then took a look at the nasal conchae. These consist of the superior, middle and inferior conchae. The most superior air channel is called the sphenoethmoidal recess. Below each conchae, there is a superior, a middle and inferior meatus.

The crista galli of the ethmoid bone is where the falx cerebri attaches to the skull. On either side of the crista galli is the cribriform plate and here, the foramina peppered through the plate are vital to olfaction and contribute to the flow of air through your nasal cavities along with the olfactory epithelium, olfactory bulbs, olfactory tract and olfactory cortex of the brain. The orbital plate of the ethmoid bone articulates with a frontal bone and the maxilla to then make up the medial wall of the bony orbit. We saw that inside the bony orbit, there are also anterior and posterior ethmoidal foramina. These foramina transmit the anterior and posterior ethmoidal vessels and nerves.

Moving inside the body of the ethmoid bone, we learned that the anterior ethmoidal air cells open into the frontonasal duct, the middle ethmoidal air cells open onto the ethmoidal bulla, and the posterior ethmoidal air cells open onto the lateral wall of the superior nasal meatus. The ethmoidal air cells receive innervation from branches of the ophthalmic and maxillary nerves.

We also saw that the ethmoidal bulla is formed by a swelling of middle ethmoidal air cells into the lateral wall of the nasal cavity. The groove extending anteriorly from the ethmoid bulla is called the ethmoidal infundibulum. The infundibulum forms the frontonasal duct channel through the ethmoidal air cells that opens into the frontal sinus. Lastly, we saw that the uncinate process is a small projection of bone that forms a small part of the wall of the maxillary sinus and articulates with the inferior nasal concha.

And that's it for this tutorial about the ethmoid bone. Thank you for watching and I will see you on the next one.

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