This is the first tutorial I’m going to do on the skull. I’m just going to do a couple of tutorials because that’s quite a lot to talk about.
The skull consists of 22 bones not including the ossicles of the ear. These bones are connected by sutures. On this model, you can see these lines here on the skull. These are sutures, which are immovable fibrous joints which connect these joints together.
The skull consists of the cranium and the mandible. The mandible is this lower jaw bone and the cranium is the rest of the skull. The cranium can be divided into an upper part and a lower part. This tutorial will just be on the upper part of the skull. The upper part contains the brain and it’s referred to as the calvaria. The lower part of the cranium is the facial skeleton, so all of these. It’s called the viscerocranium. That would be my next tutorial.
You’ve got six bones which make up the calvaria – the frontal, parietal, temporal, occipital, sphenoid and ethmoid bones, so six bones. The frontal bone, here. You can just about see the greater wing of the sphenoid – not just about, but you can see the greater wing of the sphenoid here. The rest of it, I’ll show you in a bit. The parietal bone is this large bone. Well, it’s two bones. You’ve got two parietal bones on either side, which make up a lot of the side of the skull. And you’ve got two temporal bones on either side. And then the occipital bone. You can see the ethmoid bone making up the medial wall of the orbit on both sides.
Those are the six bones that make up the calvaria.
First, I’m going to talk about the frontal bone. You can see this big bone here is the frontal bone. It’s separated from the parietal bone by this suture. This is the frontal bone, all of this. It makes up the upper margins of the orbit laterally and medially.
Just some things to point out. You can see these ridges here just above the orbit – not so clearly, but you can make them out. These arches here are called the superciliary arches. They’re prominent in males than females. And this angle here is quite deep because you can see this little depression where the superciliary arches meet. This is called the glabella.
That’s the frontal bone.
Next, I’m just going to talk about the parietal bone. You’ve got two parietal bones. You’ve got two parietal bones and two temporal bones, the rest are single bones. The occipital, frontal, sphenoid and ethmoid are single bones. Parietal bones are paired and temporal bones are paired on either side.
This big parietal bone makes up a lot of the lateral wall of the skull. You can see at the front it articulates via the suture with the frontal bone and posteriorly, it articulates via this suture with the occipital bone. Inferiorly, it articulates with the temporal bone via this suture. And it also articulates with the greater wing of the sphenoid via this suture.
I’ll just show you from the top what that looks like. You can see this suture down the sagittal plane, so it’s called the sagittal suture. The two parietal bones articulate via that.
Those are the parietal bones, big bones on the side of the skull.
The next bone I’m going to talk about are the temporal bones, which as I’ve showed you before and explained, they’re on both sides. The temporal bones, this bone here.
There are quite a few things to point out on the temporal bones. You’ve got a squamous part, which makes up part of the lateral wall of the skull and you’ve got a petrous part, which you can see inferiorly. I’ll show you that in a second.
The squamous part is just this flat bit. Squamous just means flat. You’ve got squamous cells which are squashed, flat cells. The squamous part is the large flat bit, which forms the lateral wall of the cranium. This articulates anteriorly with the greater wing of the sphenoid and posteriorly with the parietal bone and the occipital bone down here.
Just here you can see this process, which is called the mastoid process. This is quite important because a few muscles attach here, most notably the sternocleidomastoid (it has an attachment there). I’ll just show you that. You can see this big sternocleidomastoid attaches there and you’ve got this muscle here, the digastric muscle. That’s the mastoid process. And if you look from the back, you can see this little notch here, which is the mastoid notch.
Also, you’ve got this process sticking out here, which is called the zygomatic process of the temporal bone because this is the zygomatic bone and it sticks out towards that. Then you’ve got this hole here, which is the external acoustic meatus (this little hole there).
You’ve got the squamous part, the mastoid process, the zygomatic process and this external acoustic opening/meatus.
I’ll just rotate this around, so you can now see the petrous part of the skull. If you just look at the temporal bone, you can see it runs all the way around her and it’s all of this bone here. That’s the mastoid process. We’ve also got this little pointy thing, which is the styloid process.
From the inferior view, this is the petrous part of the temporal bone and this houses the inner and middle ear. We’ll just have a look at that rotating around, you can see exactly how the temporal bone works. If we face the temporal bone on the inferior aspect, it’s the petrous part.
The next bone I’m going to talk about is the sphenoid. This bone confused me quite a bit when I was learning about it. From this lateral view of the skull, you can see the greater wing here and you can see the greater wing on either side, but you’ve only got one sphenoid bone. The sphenoid connects in the middle via body, which you can’t see here. I’ll show you in a second. The sphenoid, in the middle, it’s got the body, it’s got the greater wings, which you can see laterally from outside the skull and it’s got two lesser wings (which I’ll show you and you can see them from inside the skull) and then it’s got two pterygoid processes inferiorly, which point downwards.
What you need to know about the sphenoid is that it consists of a body, two greater wings, two lesser wings and two pterygoid processes.
The best way to show you it is, again, if you follow it as I rotate the skull inferiorly, you can just keep an eye on the greater wing here. And as I rotate it around, you still see the greater wing here. The greater wing, rotating it around and you can this pointing downwards. These are the pterygoid processes. You’ve got them on both sides. See here the pterygoid processes sticking downwards. The greater wing and then the pterygoid processes pointing downwards.
What you’ve got here, you’ve got a lateral plate of the pterygoid process and a medial plate of the pterygoid process. This little hook thing that you can see is called the hamulus. In between pterygoid processes, you’ve got the body of the sphenoid, which I’ll show you from an inside view.
Just to show you again, you’ve got the greater wing of the sphenoid and then you’ve got this downward pointing process, the pterygoid process. And then you’ve got your little hook, which is the hamulus, the lateral middle plate and you’ve got the body of the sphenoid connecting in between.
I’m just going to switch to a view, this is a view inside the cranium. This now is the body of the sphenoid and this here is the greater wing. If I just show you from outside, the greater wing on the outside. And then from the inside, this is where the greater wing is. And now you can see the lesser wings. It’s not actually that clear here, but the lesser wing of the sphenoid is here.
And then just anterior to the lesser wings, you have the ethmoid bone, which again, isn’t very clear, but I’ll just point out quickly. The ethmoid bone here, you’ve got this little process pointing outwards, which is the crista galli. And then you’ve got cribriform plate of the ethmoid bone around here. You can’t see it on here because it’s not very clear, but you’ve got little, tiny holes on the cribriform plate where small branches of the nerves from the olfactory nerve penetrate through. That’s the ethmoid bone.
And just posterior, you have the lesser wing of the sphenoid. And then this is the body of the sphenoid.
Just where I showed you the greater wing, which is lateral to the body of the sphenoid, you’ve got these depressions. This is where the temporal lobe of the brain sits. I’ll just show you where the temporal lobes sit. This is just lateral to the body over here. If I bring in the brain, you can see how it sits in that depression lateral to the body.
This is looking at the body from a lateral view. We just looked at it from here and now, I’m just rotating around laterally. And you can see this little hallowing. This is called the hypophyseal fossa and this is where the pituitary gland sits. The whole structure including this little protrusion is known as the sella turcica. This anterior wall is called the tuberculum sellae and this posterior wall is called the dorsum sellae. Collectively, this makes the sella turcica. And this little ditch is the hypophyseal fossa where the pituitary gland sits. If I show you that, you can just see the pituitary gland, the pituitary stalk sitting in that little fossa. The word ‘fossa’ just means ditch in Latin. This thing just looks like a ditch.
Just to quickly recap, you’ve got the body of the sphenoid, the greater wings, you’ve got the lesser wings and then you’ve got the pterygoid processes which stick out down here pointing inferiorly, the pterygoid processes.
That’s the sphenoid.
Finally, you’ve got the occipital bone, which is this bone at the back of the skull. It consists of four parts – the squamous part, which is this flat bit at the back, two lateral parts, which sits on either side of the foramen magnum, which is this big hole here where the spinal cord comes up into the skull and becomes the brain stem and then you’ve got this bit here, which is the basilar part just anterior to the foramen magnum.
You’ve got the two lateral parts on either side of the foramen magnum, the basilar part, which is anterior to the foramen magnum and the squamous part at the back, which is the flat bit.
Just a few things to point out. Quite noticeable is this big bump here, which is the external occipital protuberance. And then you’ve got the nuchal lines, the superior nuchal lines and the inferior nuchal lines.
And if you look inferiorly, you’ve got this little raised bit of bone, which is the external occipital crest. And then the last bit to mention is these things here, which are the occipital condyles. These articulate with the first vertebra, the atlas. If I just show you that, you can see here the atlas bone articulating with the occipital condyles.
That’s the bones that make up the calvaria. The next tutorial will be on the facial skeleton, so the bones of the skull which make up the facial skeleton.