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Sphenoid bone

Structure and landmarks of the sphenoid bone.

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Transcript

Hey everyone! It's Nicole from Kenhub, and welcome to our tutorial on the sphenoid bone. In this video, we’re going to be looking at the sphenoid bone and all of its bony features. To get a good understanding of the sphenoid bone, we’ll first look at the sphenoid bone as a whole then we’ll look at the four main parts of the sphenoid bone which includes the body, the lesser wings, the greater wings, and the pterygoid processes. When we look at each of these parts, we’ll go into a little bit more detail and we’ll look at the bony features that we can find in each of these parts of the sphenoid.

The sphenoid bone is the most complex bone of the human body. Because of its shape, it’s also known as the wasp bone. The sphenoid bone makes up most of the part of the middle part of the base of the skull as we can see in this image here, and we’re looking at the skull from an anterior view. And the teeth of the upper jaw, which we can see here, are at the anterior aspect and all the other parts highlighted in green are part of the sphenoid bone.

From an internal aspect, the sphenoid bone contributes to the floor of both the anterior and middle cranial fossae. And in this image, we’re looking at the skull from a superior view where the calvaria or the skullcap has been removed. In this image, we’re looking at a sagittal section of the skull and we can see the left half of the sphenoid bone highlighted in green.

All these images we’ve looked at so far show us how far the sphenoid bone reaches in all directions, and as you can see, there’s a lot going on. In this tutorial, we’ll break it down and isolate each of the important features that this wasp bone has.

As I mentioned earlier, there are four main parts to this sphenoid bone and in this anterior view of the sphenoid, we can see the body highlighted in green in the middle, the lesser wings which are highlighted now and they project laterally from the superior aspect of the body, and now we can see the larger greater wings highlighted in green making up the most lateral part of the sphenoid bone; and finally, the pterygoid processes are what we can now see highlighted in green. And these processes project inferiorly from the body.

Throughout this tutorial, we’ll first look at the specific bony features on each of these parts of the sphenoid bone. We’ll then look at these features in an image of the skull to see their relationship to other bones and features of the skull.

So we’re going to start by looking at the bony features of the body of the sphenoid bone. And here we see the body of the sphenoid bone from an anterior view on the left and from a posterior view on the right. And the body is the most centrally positioned portion of the sphenoid bone. Anteriorly, it contributes to the nasal cavity while laterally, it builds the medial wall of the optic canal.

In this superior view of the cranial fossa, we can see where the body of the sphenoid bone is in relation to the other bones. So, anteriorly, it articulates with the ethmoid bone in the midline and the frontal bone laterally while posteriorly, it articulates with the clivus of the occipital bone. And from this view, we can also see some bony features that are part of the body. This highlighted depression in the middle of the superior aspect of the body is called the sella turcica, and the name sella turcica means “Turkish saddle” in Latin, and we can see in this image the saddle shaped depression that we’re talking about. The sella turcica houses the pituitary gland.

The anterior margins of the sella turcica seen on the left are called the tuberculum sellae with the posterior margins seen on the right called the dorsum sellae and focusing on the posterior aspect, this image shows a posterior view of the dorsum sellae.

What we can see highlighted now is a portion of the dorsum sellae called the posterior clinoid process, and from a superior view, we can see these processes highlighted in green here. The posterior clinoid processes along with the anterior clinoid process serve as attachment points for the tentorium cerebelli, which is a dural meningeal infolding. The anterior clinoid processes are part of the lesser wings so we’ll visit them again a little bit later on.

Located within the sphenoid body are the sphenoidal sinuses, and in this anterior view of the sphenoid, we can see the sinuses or spaces highlighted within the body, and these are paired sinuses so we have one on each side right and left and they split in the midline of the body of the sphenoid. In this midsagittal view, we can see the close relation of the sinuses to the nasal cavity anteriorly and the sella turcica posteriorly.

From an anterior view, we can identify two more bony features. Firstly, the sphenoidal crest which runs along the anterior aspect of the body in the midline, and the sphenoidal crest articulates with the perpendicular plate of the ethmoid bone which helps to form the nasal septum. On either side of the sphenoidal crest are the sphenoidal conchae. And the sphenoidal conchae make up the anterior wall of the sphenoidal sinuses and part of the posterior wall of the nasal cavity. The sphenoidal sinuses are able to communicate with the nasal cavity through openings in the sphenoidal conchae.

Finally, from a posterior view, we can see the sphenoidal rostrum, and this structure is a small triangular spine that is continuous with the sphenoidal crest which we saw running down the midline of the anterior surface of the body. The sphenoidal rostrum articulates with the vomer bone inferiorly.

The last feature associated with the body of the sphenoid bone that we’ll identify is the carotid groove of the sphenoid bone, and we can see it in this posterior image of the sphenoid highlighted in green. The groove runs along the lateral aspects of the body of the sphenoid bone and is a groove where the internal carotid artery lies as it courses through the cavernous sinus.

Alright next, we’ll have a look at the lesser wings of the sphenoid bone, and we can see these features from an anterior view on the top image and from a posterior view on the bottom image. And they arise superolaterally from the body where they form the optic canal. The optic canal contains the optic nerve and also the ophthalmic artery.

In this anterior view of the sphenoid bone within the skull, we can see the lesser wings highlighted in green and here we can see that they contribute to the roof of the orbit. In the superior view, we can see the superior surfaces of the lesser wings which contribute to the anterior cranial fossa.

The parts of the lesser wings that are highlighted now are features we briefly looked at earlier, so these are the anterior clinoid processes and along with the posterior clinoid processes, the anterior clinoid processes serve as attachments points for the tentorium cerebelli, which is a dural meningeal infolding.

Next, let’s have a look at the greater wings of the sphenoid bone. and we can see them from an anterior view in the top image and from a posterior view on the bottom image, and they arise posterolaterally from the body and have several surfaces contributing to various aspects of the skull, and we’ll go through these a little bit later on. Firstly, though, let’s have a look at the greater wings and where we can see them in an articulated skull.

In this anterior view of the sphenoid bone within the skull, we can see the greater wings highlighted in green and they contribute to the lateral wall of the orbit, and we can also see part of the greater wing here on the lateral aspects of the cranium. In this superior view, we can see the superior surfaces of the greater wings contributing to the middle cranial fossa.

When looking at the greater wings, we can see that they encompass quite a lot. They contribute to many different aspects of the skull, articulate with several other bones, and have several features within them, and to break it down, we’ll first look at the surfaces of the sphenoid bone and then we’ll look at the margins or the articular surfaces, and finally, we’ll have a look at the bony features within those regions.

So, there are five surfaces that make up each greater wing of the sphenoid bone and luckily they’re all named after the structure that they’re closely associated with. And they are, the cerebral surface, the orbital surface, the temporal surface, the infratemporal surface, and the maxillary surface which is also called the pterygopalatine surface. So, let’s work our way through the list.

So, the cerebral surface of the greater wing of the sphenoid bone is highlighted in green on our image of the cranial fossa, however, as we’re looking at the sphenoid bone from a superior view in this image, it doesn’t quite look the same as the image on the left. So, in the image of the cranial fossa, we’ll see that the cerebral surface of the greater wing is what makes contact with the cerebrum or what we usually call the brain. On this image on the left, we can see the dorsum sellae just here which as we mentioned is on the posterior aspect of the bone, so therefore in the cranial fossa image, we can see it here but from a superior view.

On either side are the greater wings, specifically the cerebral surface of the greater wings, which we can see highlighted in both images. So if you imagine rotating the sphenoid bone towards you by ninety degrees, then you’ll notice that we would be looking at the superior aspect of it, and that’s what we can see in this image of the entire cranial fossa.

In this image, we’re looking at the sphenoid bone from an anterior view, and what’s highlighted is the orbital surface of the greater wing. Looking at the sphenoid bone in the context of the entire skull, we can see that this orbital surface is what makes up the lateral wall of the orbit.

The surface highlighted in this anterior view of the sphenoid bone is the temporal surface of the greater wing. In this image of the entire skull intact, we can see the temporal surface on the lateral aspect of the skull closely associated with the temporal bone, the temporalis muscle, and the temporal fossa.

The next surface we can see highlighted in this anterior view of the sphenoid bone is the infratemporal surface of the greater wing. So, looking at the inferior aspect of the skull, we can see the infratemporal surface highlighted here where it makes up the roof of the infratemporal fossa.

The last surface of the greater wing is the maxillary surface which is highlighted in green on this anterior view of the sphenoid bone. And it’s closely associated with the maxillary bones anteriorly and forms the posterior boundary of the pterygopalatine fossa.

So, there are three margins of the greater wings, and on an isolated sphenoid bone, these margins look somewhat serrated. They are the areas of the sphenoid bone that articulate and forms sutures with other bones of the skull and luckily again, they are named for the bone that they articulate with. And the three margins of the greater wings are the parietal margin, the squamosal margin, and from the anterior view, we can see the zygomatic margin.

In this image of the posterior aspect of the sphenoid bone, the areas highlighted in green are the parietal margins of the greater wings. And in contrast to the smooth surfaces we looked at previously, we can see that this margin is rough and serrated in appearance. And this margin as its name suggests is the articulation point for where the sphenoid bone meets the parietal bone.

So, this image shows us a lateral view of the skull with the sphenoparietal suture highlighted in green, and this over here is the sphenoid bone and over here, we have the parietal bone. Where they meet is where the parietal margin of the greater wing of the sphenoid bone articulates with the parietal bone.

The squamosal margin which we can see now highlighted in green articulates with the squamous portion of the temporal bone. And this articulation forms the sphenosquamosal suture which we can see on both the external and internal surfaces of the skull in these images.

Finally, we’re looking at the zygomatic margin of the greater wing of the sphenoid bone, and since we can see the sphenoidal crest here, we know that we’re looking at the sphenoid bone from an anterior view. As I’m sure you’ve guessed, the zygomatic margin of the greater wing articulates with the zygomatic bone forming the sphenozygomatic suture. And looking at the lateral aspect of the skull, the sphenozygomatic suture is located deep to the frontal process of the zygomatic bone. And then from an anterior view, we can see the sphenozygomatic suture on the lateral aspect of the orbit.

The final aspect of the greater wings that we’ll look at are the bony features that are within these various surfaces and margins that we just spoke about, and in this anterior view of the sphenoid bone what we can see highlighted in green is the infratemporal crest. And the infratemporal crest is a crest or a ridge where two surfaces meet. And these surfaces we’ve already identified, so superiorly, we have the temporal surface and inferiorly we have the infratemporal surface.

In this lateral view of the skull, we can see the infratemporal crest being a landmark dividing not only the surfaces of the greater wing of the sphenoid, but also of the temporal fossa which is here, and the infratemporal fossa which is over here.

So, we have a couple of images here. We have an anterior view on the left and a posterior view on the right, and in both of those images, the structure highlighted in green is the spine of the sphenoid bone. On the posterior view, we can see the spine projecting from the inferior portion of this margin which is the squamosal margin. And looking at the anterior view more closely, we can see how the spine of the sphenoid projects inferiorly as well. And on this portion, a ligament attaches from the spine of the sphenoid to the lingula of the mandible, and this ligament is called the sphenomandibular ligament.

Looking at the inferior view of the skull, we can see the relationship between the spine of the sphenoid and different features of the temporal bone, and since it is at the end of the squamosal margin, it articulates with the squamous part of the temporal bone as well as the petrous part of the temporal bone. And it fits right into the angle between these two portions of the temporal bone.

The next feature highlighted in green is seen on the posterior aspect of the sphenoid bone and this is called the sulcus for the auditory tube, and its purpose is just for that. The cartilaginous auditory or the pharyngotympanic tube runs in the sulcus.

So within the greater wings of the sphenoid bone, there are a few foramina or holes which allow for various nerves and vessels to travel in and out of the skull and the foramina highlighted in green in these images are called singularly the foramen rotundum. And the foramen rotundum is a little bit more easily seen in this superior view of the sphenoid bone. The maxillary nerve which is the second division of the trigeminal nerve or cranial nerve five travels through the foramen rotundum.

And the next foramen which is now highlighted is the foramen ovale, and it gets its name from its oval shape. And passing through the foramen ovale is the mandibular nerve which is the third division of the trigeminal nerve.

The most lateral foramen in the greater wing of the sphenoid bone is the small one highlighted now and this is known as the foramen spinosum. And it gets its name due to its close proximity to a bony feature we looked at recently which is known as the spine of the sphenoid bone. The middle meningeal artery travels through the foramen spinosum to supply the dura mater surrounding the brain and the calvaria.

So the spaces in this posterior view of the sphenoid are called the superior orbital fissures, and they’re not really a hole in the bone like the previous foramina. Rather, they’re the spaces between the greater wing and the lesser wing on each side. And we can see the superior orbital fissures in this anterior view of the sphenoid bone as well. And this space is where many structures, nerves and arteries travel through to get from the cranium to the orbit. In this image is a zoomed-in view of the right orbit from an anterior view, and the superior orbital fissure is highlighted in green.

Alright, of the four main parts of the sphenoid bone, so far we’ve looked at the body, the lesser wings, the greater wings and all of their associated surfaces, margins, features, and foramina. So, lastly, we’re going to be exploring the bony features of the pterygoid processes to complete our picture of the sphenoid bone.

So, the pterygoid processes are inferior bony extensions that descend perpendicularly from the junction of the greater wings and the base of the sphenoid body, and we can see these processes in both the anterior view of the sphenoid bone and also in the posterior view. In this image of the posterior skull, we can see the most inferior portions of the pterygoid processes highlighted in green, and looking at the skull from an inferior view shows us most of the posterior aspect of the pterygoid processes.

Alright, so the pterygoid processes contain two pterygoid plates each, and the first that we can see highlighted in these images are the broad lateral pterygoid plates. The other is the more medial and narrow plate called the medial pterygoid plate. Focusing on the lateral plate first, we can see it in this inferior view of the skull as the most laterally situated plate of the pterygoid process and it forms the medial wall of the infratemporal fossa which is in this area and is the origin of both the medial and lateral pterygoid muscles.

The medial pterygoid plates are of course are more medially situated than the lateral ones which means in this sagittal section of the skull, we can see the medial side of the medial pterygoid plate very clearly and that’s highlighted in green. In this inferior view of the skull, we can once again see the medial pterygoid plates highlighted in green.

In this next image, only the inferior ends of the medial pterygoid plates are highlighted and this inferior hook-like projection is called the pterygoid hamulus. The pterygoid hamulus acts as a pulley for the tendon of the tensor veli palatini muscle, and the origin and location of this muscle belly are related to bony features we’ll look at shortly.

On the posterior aspect of the pterygoid processes, the lateral and medial pterygoid plates form a depression or fossa between them, and this depression is simply called the pterygoid fossa. And in this fossa is where we’ll find the muscle belly of the tensor veli palatini muscle and also within the pterygoid fossa is the medial pterygoid muscle which originates on the medial aspect of the lateral pterygoid plate so that the belly is then within the pterygoid fossa.

Superior to the pterygoid fossa is another depression called the scaphoid fossa of the sphenoid bone and in this fossa is where we’ll find the origin and the muscle belly of the tensor veli palatini muscle. The origin of the tensor veli palatini muscle is the scaphoid fossa, but remember in the previous slide, we saw that the muscle belly lies within the pterygoid fossa. Inferiorly, the tendon of the tensor veli palatini muscle wraps around the pterygoid hamulus as it runs to the soft palate.

Inferior to the pterygoid fossa, there is a space between the medial and lateral pterygoid plates and we can see it highlighted in green in both the anterior and posterior views of the sphenoid bone, and this space is called the pterygoid notch. Fitting in the pterygoid notch is the pyramidal process of the palatine bone, and we can see this articulation highlighted on this inferior view of the skull.

The final feature that we’ll look at today is the pterygoid canal which is highlighted in both the anterior and the posterior views of the sphenoid bone, and the greater and deep petrosal nerves merge to form the nerve of the pterygoid canal which along with the similarly named artery and vein travel through the pterygoid canal. And these structures course through the canal to travel from the middle cranial fossa to the pterygopalatine fossa.

So, there you have it – the big and small features of the sphenoid bone. And as we’ve seen, the sphenoid bone is associated with many different aspects of the head so therefore it would follow that there’s a lot of clinical correlates related to the sphenoid bone and we’re going to talk through one interesting example.

So do you remember when we spoke about the sella turcica? This depression in the body of the sphenoid bone houses the pituitary gland. And in the case of a pituitary gland tumor, surgery is often the best and most common treatment. So instead of opening up the cranium in trying to access the pituitary glands through the rest of the brain, surgeons are able to operate from an anterior approach, and they connect us to the pituitary gland by placing their tools through the nasal cavity through the sphenoidal sinus and through the sella turcica. And this minimally invasive surgery is called endoscopic transsphenoidal surgery.

So, now, you’re an expert on the sphenoid bone, but before I’ll let you go, let’s have a quick run-through what we looked at today.

So we began by looking at the sphenoid bone as a whole and all the aspects of the skull that it contributes to. It makes up the lateral wall of the orbit, most of the middle portion of the base of the skull, and contributes to the anterior and middle cranial fossae.

We then broke the sphenoid bone down into four main parts – the body, the lesser wings, the greater wings, and the pterygoid processes. And within each of these parts, we looked at the surfaces, the margins, the foramina, and other bony features that make up the sphenoid bone as complex and as interesting as it is.

Alright, so that brings us to the end of our tutorial on the sphenoid bone. I hope you enjoyed it. Thanks for watching and happy studying!

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