Hello anatomy colleagues this is Dr. Alsup and it is my absolute pleasure to get to discuss
the general osteological features we need to know for this session. This will be a first
where I think the osteology may actually be our longest dissection video. And it's not
just because osteology is my favorite which it is but because there are so many important foramina
that we need to be familiar with. Particularly with the neurovasculature that traverses this
region. The focus on this video will be on being able to actually identify and distinguish these
structures and a little less so as to what goes through these structures as it's considered in the
learning objective. But first I want to get started with some basic terminology that you will hear
us use quite often, and these are calvaria and basicranium. So just to get our bearings we are
looking at a lateral view of the skull. And we know this is the skull. The term skull means the cranium
which is everything except the mandible. This right here is the mandible or the lower jaw so
when you have the mandible included with the rest of the cranium then you are looking at the skull.
The cranium is composed of the facial region, all right, so all these kind of facial bones right here
just kind of a generalized location. As well as the neurocranium and the neurocranium is basically
everything else. It is the portion that will protect the brain. and the neurocranium is further
subdivided into these two basic areas that we want you to know. There is going to be the cranial
base so anything kind of in this bottom portion inferior extending towards this region as well.
And the cranial base is often referred to as the basicranium that's the actual term for it and
that's where the brain sits. And this is actually where we'll spend the majority of our time in this
video as this is where the cranial fossae are located as well as the majority of the foramina.
The other portion is called the calvaria and you know this is kind of give or take, this
is not a very set line, but the calvaria is often called the roof of the cranial vault
or more crudely we refer to this as the skull cap. And it's the calvaria that we will be removing in
the craniotomy to gain access to the superior portion of the brain. so sometimes it can be a
little bit more superiorly placed in terms of this line. There's a little
variability in terms of what you'll see in different sources.
All right now let's move on to a view that we will be using quite a bit.
And in this view that calvaria or the skull cap has been removed as well as of course the
brain and associated meninges and we are looking superiorly at an internal view of the basicranium.
So this is where the brain would have sat during life. And as we know from the learning objective
this area can be divided into three cranial fossa which are arranged in kind of a stair step
fashion from the shallowest anterior cranial fossa to the deepest posterior cranial fossa.
And I always know I'm looking posterior because of this big hole right here the foramen magnum
it's always going to be posteriorly placed so I know this direction is anterior.
The anterior cranial fossa spans from the frontal bone specifically the squamous
portion or this anterior flattened portion back to the lesser wings of the sphenoid.
So the lesser wings are going to be right here, this is a lesser wing and this is a lesser wing,
and so this whole area right here is going to be considered part of the anterior cranial fossa.
Recall that there are some differences between resources whether this medial most portion is
included within the anterior cranial fossa sometimes you see it with the middle but for
our purposes we do include this portion in the anterior cranial fossa so anterior cranial fossa
is here. The middle cranial fossa extends from the inferior portions of the lesser wings of the
sphenoid to the apices of the petrous part of the temporal bone. So this whole region right here is
the petrous portion of the temporal bone. And the apex is kind of the top or most superior portion
of this petrous part so over here would be the apex on this side. And so this middle cranial fossa
will be between the lesser wings to the apex of the petrous part of the temporal bone.
And lastly the posterior cranial fossa extends from those same apices of the petrous part of the
temporal bone all the way back to the occipital bone specifically the squamous portion of the
occipital bone or kind of the back of the head region. So posterior cranial fossa is located here.
Okay let's turn our attention to the foramina associated with these fosses
starting with the anterior cranial fossa. And let's hone our attention to
this small median bone right here in the middle referred to as the ethmoid bone.
The ethmoid is so wonderfully complex as you can see just in this small
area how much is going on there. And running right in the middle I'm kind of outlining it right here
is a projection referred to as the crista galli. And this is actually the anterior attachment site
of the falx cerebri so remember that largest dural partition that sits in the longitudinal cerebral
fissure will anteriorly attach here. And running all over the place next to the crista galli
are these small foramina and these are referred to as the cribriform foramina which will transmit
olfactory axons of cranial nerve I, so associated with cranial nerve I. Moving a bit posteriorly
we get to the sphenoid bone which is my all-time favorite bone and that is saying a
lot because I love all of them very very much. But one of the biggest characteristics of the
sphenoid are the two sets of wings. We pointed out the lesser wings in the last slide but here
they are again here are your lesser wings and you can see kind of where the suture is
for the sphenoid here. You can see how we have kind of this butterfly shaped and this
region right here is associated with the greater wing of the sphenoid.
Just medial to the lesser wings of the sphenoid there will be the optic
canals and I'm going to circle the optic canals here. And these are going to lead into or away
from the orbit so the ophthalmic artery as well as cranial nerve II will traverse the optic canals.
All right let's continue our exploration of the sphenoid bone but now let's move into the middle
cranial fossa so just under these lesser wings of the sphenoid so almost completely occluded
from view. You can see a little peak right here of the superior orbital fissure. And again as
mentioned this is going to be inferior to the lesser wings of the sphenoid. We will see the
superior orbital fissure in an anterior view in a few slides which you can see quite a bit better.
Lots of neurovasculature heading towards the orbit or away from the orbit here so
you're gonna have, oops excuse me, you'll have cranial nerves III,
VI, as well as VI that are going to traverse this region. And then you will have the sensory
one of the sensory divisions of the trigeminal nerve or v1
the ophthalmic nerve traveling through there.
Okay just posterior to those optic canals that we saw associated with the anterior cranial fossa
you will have this divot of bone it's kind of like a saddle sitting in a saddle region.
This is referred to as the hypophysial fossa and it is going to
accommodate a portion of the pituitary gland. So sometimes you see this with an e here same
thing "hypophyseal fossa" so that's where that or a portion of the pituitary gland will sit and
as you can see it's very well protected by bone as that's a very important gland in that region.
All right lateral to the hypophysial fossa and a component of the greater wings of the sphenoid
is a trio foramina or a paired trio of foramina. The more anterior and almost perfectly rounded one
is what was is referred to as the foramen rotundum. And the foramen rotundum allows the
maxillary nerve or the second division of the trigeminal nerve to traverse and exit the skull.
Just posterior and slightly lateral to the foramen rotundum is the foramen ovale. So here are the
foramina ovale, and the foramen ovale is going to allow the third division or the mandibular nerve
the third division of the trigeminal nerve often referred to as the mandibular nerve
to exit the skull. v3 is larger than v2 thus the foramen ovale is larger. Lastly posterior lateral
to the foramen ovale is this little pin prick of, and I should really draw this right here,
this little pin prick right here is going to be the foramen spinosum so a much smaller
foramen. And that's because you have the relatively small middle meningeal artery
which is ascending to enter the skull to supply the meninges. So rotundum, ovale,
spinosum. Okay now let's focus on the posterior cranial fossa.
Again recall that this whole region right here I'm going to outline it on this side
is referred to as the petrous part of the temporal bone. It's a rock-like portion and we have that
apex right here which is that demarcation between the middle and the posterior cranial fossa.
On the petrus part they're going to be two openings leading into the internal acoustic
meatus so it's in that general region there it's not quite as easy to see on
this side but it would be right about here.
And this leads into the deeper portions of the petrous part of the temporal bone. And within
the petrous part of the temporal bone you're going to have both the middle and the inner ears.
The facial nerve as well as the vestibulocochlear nerve so cranial nerve VII and VIII
those are going to enter in to the internal acoustic meatus. Facial nerve will eventually
make its way out of the skull or most of the facial nerve will, whereas vestibulocochlear
goes into the petrous part heading towards that inner ear region and kind of sticks around there.
All right next let's move to the jugular foramen. The jugular foramen is actually
located between the temporal bone and the occipital bone and as you can see here as I'm
outlining them they are very irregularly shaped they don't even look the same on either side.
And that is because we do have those dural venous sinuses draining in this region and that's where
the internal jugular vein begins. And so veins have varying pressure, therefore we have this variably
shaped foramina right here. Additionally you're going to have cranial nerves IX, X,
and XII traversing the jugular foramen. So glossopharyngeal, vagus, and the accessory nerve.
So much going on with the jugular foramina. Moving to the occipital bone there is this
dominant hole that I think is pretty easily distinguishable
that is the foramen magnum. And it's at the foramen magnum that the medulla oblongata will transition
to the spinal cord. Additionally the accessory nerve will ascend through the foramen magnum
to almost immediately exit through the jugular foramen so you do have the accessory nerve
associated with both the jugular foramen and the foramen magnum. Also those vertebral arteries will
enter the skull via the foramen magnum to be one of the dominant suppliers for the brain. And
lastly close to the foramen magnum and a bit more anteriorly placed you have the hypoglossal canals.
And this is the means by which the hypoglossal nerves will exit the skull.
Now as these foramina are transmitting neurovasculature it makes sense that we
will be able to see some of those structures we've just discussed in an inferior view as well which
is what we're looking at. And here we're looking at a cranium not a skull because the mandible is not
present so this is upper jaw maxillary region. A few structures you can see well in this view
include the foramen ovales which you can see on either side as well as the foramen spinosum
right here and right here. You can't really see the foramen rotundum very well it's hidden here
by the pterygoid plate of the sphenoid so it'd be kind of tucked up under that region.
Moving posteriorly we can see the external openings of the carotid canal right here.
And so this is the means by which the internal carotid artery will enter the skull.
And there's a very complex directionality once we get past that region but for our
purposes we do want you to know at least this general region where it enters the skull.
And lastly of course hopefully you're starting to feel familiar
with the large foramen magnum which is clearly visible from an inferior view as well.
And before we leave our conversation on the osteology of this session I did want to look
at this anterior view with a focus on the orbit. And two of the openings that we have discussed are
also visible from this view. This almost perfect circle right here is going to be the optic canal,
which you can see very clearly almost perfectly rounded whereas we have this more
irregularly shaped fissure right here which is the superior orbital fissure. And so recall
we couldn't see the fissure very well from that superior view of the internal basic cranium but
you can very clearly see the size of the superior orbital fissure here so quite visible when you're
looking into the orbit from an anterior view. And you have some other foramina and fissures
in this region which will obviously transmit neurovasculature as well but these are the two
that we want you to focus on. All right excellent thank you for working through the osteological
structures of the cranial cavity and brain session. Take your time to review and always feel free to
reach out to me or any of my anatomy colleagues with questions. Have a great rest of your day