Hello, hello, everyone. This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, I'm going to be talking about the inferior view of the base of your skull.
Now, this, in my opinion, was one of my biggest fears when I was learning in medical school. When I looked at the inferior view of the skull or the inferior view of the base of the skull, I was petrified. And the reason was due to the fact that there are so many little structures that you have to remember and memorize and what passes through them. And the intention of this tutorial is to clarify all these structures and make it easier for the next… for your next exam.
So what I'm going to be doing is focusing on different structures that we’re going to be seeing on the inferior view of the base of the skull which is when you just turn the skull and look at the inferior view of the skull, the entire cranium, and removing, of course, we’re removing here the mandible. And now we’re going to look at the different structures which include the bones, the bony structures or bony landmarks associated to those bones, and most importantly and the trickiest part, the foramina and the fissures—so all of these little holes and trying to explain you what is going to go through these fissures and through this foramina.
Now, let’s start with the very first topic here, the bones that we find on the inferior view of your skull. And the first one that I'm going to highlight here, found on the anterior view, and I mean found on the anterior portion of the skull or the base of the skull, this is the maxilla. And the maxilla, from this aspect, we see the inferior aspect of this bone which is essentially your upper jaw which houses the upper teeth. And it plays an important role in mastication or chewing, when you chew things, and also during communication.
The next bone in line is going to be this one here occupying most of the area of the inferior view of the base of the skull. This is known as the occipital bone. And this is an unpaired bone covering the back of your head. It is convex externally as you can clearly see here on this image. But if we were to go inside or look at the superior view of the base of the skull, looking at the inside of the base, we could see that this bone is then concave.
Now, the occipital bone is divided into a few parts that we’re not going to discuss here on this tutorial, but they’re all arranged around this foramen here, this large foramen, known as the foramen magnum, and we’re going to talk about it later on on this tutorial.
Next one is going to be this bone here. This is known as the palatine bone. This is a paired L-shaped bone that forms part of the nasal cavity and the hard palate. It is located between the maxilla and the sphenoid. And the palatine bone consists of different parts, including the horizontal plate, which forms the posterior portion of the hard palate of the oral cavity. Another part is going to be the perpendicular plate, which contributes to the lateral wall of the nasal cavity, and also the pyramidal process, which comprises the lesser palatine canals, and here, the lesser palatine nerves run through.
The next bones—I say bones because these are two bones—the parietal bones, one on each side of your skull, seen here highlighted in green. And what happens is that they’re found on either side of the neurocranium. And these bones have concave internal and convex external surfaces.
The next bone is one that is shaped as a wasp, or a butterfly to be more beautiful I would say. This is the sphenoid bone. And the sphenoid makes up most of the middle part of the base of the skull, as you can see here, and contributes to the floor of the middle cranium fossa.
This bone is comprised of four parts, I would say. The first one is the body that you cannot clearly see here from this, the inferior view of the base of the skull. We can see it a bit more when we look at the superior view. The other part that is not very clear as well from here is the lesser wing, but we can see the greater wing—you notice here the greater wing of the sphenoid—and also the pterygoid processes, which we, also, are going to talk about. They’re clearly seen from the inferior view of the base of the skull.
The next bones that we’re going to also be covering are known as the temporal bones. And from the inferior aspect of the base of the skull, we get a good view of these bilaterally symmetrical bones. The temporal bones also are also comprised of four parts that we’re not going to go into much detail, but it’s important to clarify here when we talk about these structures. This is the squamous part, the tympanic part, the styloid process, which you can clearly see here from the inferior view of the base of the skull, and the petrous part. And from this aspect, we can see the zygomatic process, here, of the temporal bone, which then will articulate with the temporal process of the zygomatic bone. And this is forming then this arch here, the zygomatic arch.
The next bone that we’re going to be talking about, also the next bones, these are known as the zygomatic bones. And these three or the zygomatic bones protrude laterally and form eminences on your face that we commonly call them your cheeks. So these are your cheek bones. And for this reason, they are prone to fractures, very sensitive bones to fractures.
Now, the zygomatic bone also has three processes that I would like to just briefly mention, so you have it in your mind that this bone is comprised of the frontal process, which you cannot clearly see on… from this view of the bone. But you also have temporal processes, which you can see here clearly, and we talked about on the previous slide. These are the temporal processes of the zygomatic bone, articulating here with the zygomatic process of the temporal bone and forming the zygomatic arch. And another part that we can see is the maxillary process which is articulating, then, with the maxilla. And these three processes vary in shape and size.
Next bone that I'm going to be covering—and this is the last one on our list—is known as the vomer. And this is a singular bone that runs vertically within the nasal cavity, separating the left and right sides. It articulates with the palatine bone, the maxilla, the ethmoid bone, and the sphenoid bones, while the posterior border is not attached to any bone but rather to soft tissue.
Now that we’re done covering different bones that we can find on the inferior view of the base of the skull, now, we’re going to talk about the bony structures associated to those bones that we talked about. And we’re going to start off with one that I already mentioned. This is the zygomatic arch. And the zygomatic arch is formed by the zygomatic process of the temporal bone, as I mentioned here, and also the temporal process of the zygomatic bone like I mentioned. Now, there is a suture here. This suture is known as the temporozygomatic suture which connects the zygomatic bone to the zygomatic process of the temporal bone.
The next structure that we’re going to talk about is actually paired and is known as the pterygoid hamulus. And this refers to a bilateral extension of the medial pterygoid plate of the sphenoid bone at the base of the skull, and you can clearly see here the medial pterygoid plate on each side and the lateral pterygoid plates as well. And this is the bilateral extension here, this pterygoid hamulus that has importance because the tendon of the tensor veli palatini curves around the pterygoid hamulus. And also, another importance for this structure is that the superior part of the pterygomandibular raphae also originates from here.
Next structure in line is going to be one that I already talked about briefly is the lateral plates of the pterygoid process, seen here highlighted in green, right next to the medial plates and the hamulus that we talked about. The lateral plates of the pterygoid process form the medial wall of the infratemporal fossa, while the medial surface of the lateral plate of the pterygoid process forms part of the pterygoid fossa. Structures will be attaching here, the lateral pterygoid muscle, the medial pterygoid muscle attach on the lateral and medial infratemporal and pterygoid fossa, respectively.
We already mentioned them. We talked about the hamulus that is an extension of these plates, the medial plates of the pterygoid process. The medial plates of the pterygoid process forms the lateral part of the choana and its medial surface and part of the pterygoid fossa on its lateral wall. It is longer and narrow than the lateral pterygoid plates and forms the pterygoid hamulus like I mentioned before at its lowest point.
Next structures that we’re going to be seeing here highlighted in green are found on the temporal bone. And these protrusions are known as the mastoid processes. The mastoid processes are located behind the acoustic meatus as you can see here—one on each side, the acoustic meatus—and then behind it, you find the mastoid process.
Let’s move on to the next one. This one is known as the palatine process of the maxilla. The palatine process of this bone is a horizontal extension consisting of the roof of your mouth on the floor of the nasal cavity.
Followed by this structure, we’re going to look again at the two medial plates. Now, we’re looking at the whole structure, the pterygoid process. And the pterygoid process of the sphenoid bone is comprised, like I mentioned before, of the medial pterygoid plate and the lateral pterygoid plate. And if you remember correctly from previous slides, we have the medial and the lateral. Don’t forget this. It’s very important when you’re looking at the inferior view of the base of the skull.
From this aspect, we can see that the pterygoid processes descending perpendicularly on either side, from points… from the points where the greater wing and the body of the sphenoid bone unite.
Next structure that we’re going to be seeing here, we also talked about briefly when the zygomatic bone is connecting to the maxilla. The maxilla is connecting to the zygomatic bone via the zygomatic process, which is highlighted, now, here in green. The zygomatic process of the maxilla grows laterally and meets the zygomatic bone. Superiorly, it articulates with the zygomatic bone, while posteriorly, it forms part of the infratemporal fossa.
Next structures that we’re going to be talking about… or next structure is known as the transverse palatine suture. And this suture separates the two palatine processes of the maxilla from the palatine bone. So as you remember, this is the palatine bone, and we have here the two palatine processes of the maxilla and seeing here clearly the sutures separating these structures.
If you look closely, there is another suture here that we can highlight in green. This is known as the median palatine suture. And this suture is where the two horizontal palatine processes of the maxilla that you can find here, one and two. This is where they become fused during development. And when these two plates fail to fuse properly, a developmental abnormality occurs known as cleft palate.
Next structures in line are going to be these two here known as the mastoid notches. And you can see here that they’re found medially to the mastoid processes that we also talked about. They’re also found on the temporal bone. So this is a depression that we find medially to the mastoid processes. It is also sometimes referred to as the digastric fossa as this is where the anterior belly of the digastric muscle will attach to.
Other structures, another pair here known as the choana, individually or singular for choanae. And these bilateral openings of the posterior nasal aperture are located between the nasal cavity and the nasopharynx. Now, the two openings are separated by this bone here that we just talked about before, the vomer.
Next structures are going... or the next structure is going to be the external occipital crest seen here. The external occipital crest is a ridge located on the occipital bone and is where the nuchal ligament attaches to. And you notice here that this structure is between or is located between the external occipital protuberance, which is the structure here, and also this foramen here, the foramen magnum.
The next structure covering here on our list of structures on the inferior view of the skull is a line. It is known to be as the inferior nuchal line. And the inferior nuchal line runs inferior to the superior nuchal line which is this line here that we can also see. And the muscle known as the semispinalis capitis will be inserting just above this line.
We also have another line here that we need to cover that can be seen from the inferior view, and this one is known as the superior nuchal line. And the superior nuchal line extends laterally from this structure here, this landmark known as the external occipital protuberance that we talked about before on different slides. And the superior nuchal line is an important landmark because several muscles are going to be attaching here, including these that I'm listing: the occipitalis, splenius capitis, trapezius, and the sternocleidomastoid.
Now, that we’re done covering the different bones and landmarks, it is time for us to move on to the foramina and fissures and see what’s going to pass through these… these holes. Now, the first one that we see here or two, they are known as the stylomastoid foramina. Foramina is plural for foramen. And they are located on the temporal bone between the styloid process, as you can see here—this is the styloid process—and this process here known as the mastoid process. And the stylomastoid artery and the facial nerve pass through this foramen... foramina.
Next one in line is going to be these two here, also a paired structure known as the foramen spinosum. And the foramen spinosum encircles the middle meningeal arteries and also the meningeal branch of the mandibular division of the trigeminal nerve.
The next foramina are found on the palatine bone and are known as the lesser palatine foramen, or foramina for plural. And these foramina house the lesser palatine nerves and also vessels.
If we have a lesser palatine foramina, we have to have then a greater palatine foramina. And they are located anterior to the lesser palatine foramina on the palatine bone. And the greater palatine nerves and also vessels pass through these structures.
Next one on our list is going to be these two here known as the foramen ovale. And found on the greater wing of the sphenoid, the foramen ovale transmits different structures, several structures actually, that you need to remember for your next exam. And these include the mandibular division of the trigeminal nerve, and also the accessory meningeal artery, the lesser petrosal nerve, and the emissary nerve. So all these structures that you need to write down and take it to the next exam when they ask you, “What structures pass through the foramen ovale?”
The other foramina that we’re going to be learning about are these that you find on the temporal bone. They are known as the mastoid foramina, or foramen, singular. They are found on the mastoid portion of the temporal bone. The mastoid foramina contain the emissary veins and, sometimes, branches of the occipital arteries.
One of this foramen that I’ve been talking about, the largest of them all, known as the magnum, meaning the largest or greatest, and foramen magnum is what you’re seeing now, highlighted on the image. This is the largest foramen, as I mentioned, in the entire cranium and is found on the occipital bone. The medulla oblongata, the vertebral arteries, the spinal roots of the spinal accessory nerves enter and exit the skull through this point.
From the greatest of them all, we move on to the next one known as the foramen lacerum. And this foramen does not carry any vessels. Instead, it is filled with fibrous cartilage and has an anterior opening for the pterygoid canal and a posterior opening for the carotid canal. It is located between the greater wing of… and body of the sphenoid bone, as well as the petrous portion of the temporal bone and the basilar portion of the occipital bone. So a few landmarks that you need to just remember when we look at the foramen lacerum.
Moving on from foramina, we’re going to talk about a few canals now. And we’re looking at the carotid canal. And found on the petrous portion of the temporal bone, the carotid canal is going to... this is a part where the internal carotid arteries and the internal carotid nerve plexus will pass through. And note that the internal carotid plexus contains a sympathetic nerve fibers.
The next set of structures are going to be also a… two canals. And these are known as the condylar canals. They’re found behind the occipital condyle, and the condylar canal is where the emissary vein and the meningeal branches of the ascending pharyngeal artery pass through.
Next one to cover here on this tutorial and seeing from the inferior view of your skull is going to be a fissure known as the inferior orbital fissure. And the inferior orbital fissure transmits the maxillary division of the trigeminal nerve, the zygomatic nerve, and the infraorbital vessels. This structure is found between the greater wing of the sphenoid as you can see here on this image, and also the maxilla here, and the orbital portion of the palatine bones. But we cannot clearly see here from this view.
Next ones on our list are going to be two fissures seen now, highlighted in green. These are known as the petrotympanic fissures. And here, we’re going to see here that these structures contain the chorda tympani branch of the facial nerve and also the anterior tympanic artery. It is located on the temporal bone as you can see here on the inferior view of the skull.
We’re going to move on to this one here, now that we’re looking at a zoomed-in maxilla where we can find the next structure, the incisive foramen. The incisive foramen is found on the palatine process of the maxilla, and it transmits the nasopalatine nerve, the sphenopalatine artery.
Last foramen that we need to cover for all the inferior view of the skull, this is the jugular foramen, and it is located behind the carotid canal at the base of the skull. And several structures pass through the jugular foramen. And those include the internal jugular vein, the inferior petrosal sinus, the posterior meningeal artery. And there are three cranial nerves that also are transmitted through the jugular foramen. And these include the glossopharyngeal nerve, the vagus nerve, and the accessory nerve.