Hello everyone, welcome back to SotonBrainHub.
Today, I would like to do a little presentation on the foramina of the skull base.
Now, when I say the word “foramina”, I mean a group of holes in the skull, where
structures, like arteries, veins, nerves, go through.
The term “skull base” refers to the region of the skull, which is exposed when I take
off the calvarium of the skull and take a look inside in a superior view – here.
This is one of the many topics students studying head and neck anatomy may get confused with,
as I have done so for myself.
Hopefully this presentation will give you an insight into approaching this subject with
a logical and systematic manner.
So this is a picture I took of my skull model and the first thing I’ll do is divide the
skull base into thirds.
The first of the third, marked with my red pen, is something called the anterior cranial
fossa.
The middle third, with my blue pen, is called middle cranial fossa.
And he final third is called the posterior cranial fossa, with my green pen.
The anterior cranial fossa is made up of the ethmoid bone, the orbital plate of the frontal
bone, and the lesser wing of the sphenoid bone – here.
The middle cranial fossa is from the lesser wing of the sphenoid to this ridgy part of
the temporal bone, which is the petrous part of the temporal bone.
The posterior cranial fossa will occupy the area posterior to the petrous part of the
temporal bone.
I will now go through each of the cranial fossae and talk about the relevant structures
in those regions and compartmentalising it in such way so that you remember things better.
So let’s look at the anterior cranial fossa – here’s a close up view of the anterior
cranial fossa.
The first structure you’ll come across is this protruding structure in the midline.
This is called the “crista galli” and in latin it apparently means the headpiece
of the chicken.
So this is my horrible drawing of a chicken and here’s the headpiece, OK?
Lateral to the crista galli lies a region with many holes in them.
This region is called the cribriform plate – here.
And this is where the nerve fibres of the olfactory bulb, which is a part of the olfactory
nerve (CNI), travels through.
And the olfactory bulb will sit on top, as such.
It will carry the sensation of smell via these nerve rootlets, back to the olfactory bulb,
and into the brain.
Now, if you look a bit more posteriorly, you will see another pair of protruding structures
this time extending to the middle cranial fossa, called the anterior clinoid processes
– one on either side.
Medial to that, you will see a hole where the optic nerve comes out through – aptly
named optic canal.
And this is where the visual information will be carried through and the nasal hemiretinal
fibres will cross over or deccusate at this point while the temporal hemiretinal fibres
will continue ipsilaterally.
This brings me nicely to the middle cranial fossa.
The middle cranial fossa contains many foramina so let’s look at the one in the midline
first, which is here.
In this indentation lies a really important structure of the brain called the hypothalamus,
which our primary brain region responsible for secretion of many hormones like ACTH to
trigger stress response, FSH/LH which are important for our reproductive system, and
many more.
And rom a medial view, it sort of looks like this and the hypothalamus will sit right here.
This fossa that the hypothalamus sits in is called the sella turcica, and in latin it
apparently means the Turkish Saddle.
Not sure why it had to be a Turkish saddle, but such is anatomy.
Now I have previously mentioned that the visual information from the hemiretinal fibres decusate
at this point which is directly above the sella turcica as a structure called optic
chiasma.
This means that if a patient has a pituitary adenoma, it will expand superiorly and it’s
going to compress the decussating fibres and result in a visual field deficit called bitemporal
hemianopia, which is the loss of temporal vision on both eyes.
Behind the sella turcica lies this structure, which is
dorsum sellae with two extensions on either side called the posterior clinoid processes.
For the next foramen, I will tilt the skull just a little bit so that I can see this large,
slit-like foramen – which is here.
This is called the superior orbital fissure.
A lot of structures go through this, most notably CN III, CNIV, first division of CNV
and CNVI.
This can also be visualised if you look into the orbit from an anterior view.
Now, let’s just go back to the previous diagram.
If I said the first branch of the trigeminal leaves through the SOF, that means it makes
sense structurally and logically for the other two divisions must follow suit posteriorly.
The foramen posterior to the SOF is called the foramen rotundum and this is where the
2nd division of the trigeminal, or the maxillary division exits through.
Behind that, the third division of the trigeminal, the mandibular, will exit through this oval
shaped foramen called foramen ovale.
It makes sense anatomically that it’s arranged in such way.
Posterior to the foramen ovale lies another foramen called the foramen spinosum, which
transmits a branch of the maxillary artery (which itself is a branch of the external
carotid) called the middle meningeal artery.
You can actually how the middle meningeal artery comes up through foramen spinosum and
makes these grooves here.
This vessel lies between the skull and the dura and a rupture would result in an extradural
haemorrhage.
If this is a CT scan of the skull, you’ll see an expanding haematoma which looks like
a halo of an angel here.
Now, lateral to the dorsum sellae, you will see a large foramen on each side.
This looks suspiciously large and many students think a lot of sturctures must exit through
this foramen.
The foramen lacerum, as this is called, actually does not transmit any structures.
Instead, its foramen is blocked by a fibrocartilagenous plate.
Now once again – NOTHING GOES THROUGH THE FORAMEN LACERUM!
They love asking this trick question on an exam…
If you look posterior to the foramen lacerum, you will see a small foramen with a groove
leading to the foramen lacerum.
This foramen transmits the internal carotid artery and it will travel through this foramen,
go along this group, and sit on top of the foramen lacerum and move superiorly to syphon
around the anterior clinoid processes and give off branches.
And this foramen is called the carotid canal.
So the foramen lacerum, although nothing goes through it, provides a structural base for
the internal carotid artery to sit on.
Now finally, Let’s now move to the posterior cranial fossa.
On the posterior portion of the petrous part of the temporal bone, you can see a foramen
which will lead into the body of the petrous part of the temporal bone, which is here.
This foramen is called the internal acoustic meatus and the internal acoustic meatus will
transmit the facial and vestibulocochlear nerve, CNVII and CNVIII respectively.
Looking posterior to that, you see a large foramen and this is the jugular foarmen which
not only transmits the internal jugular vein, but it also transmits the glossopharyngeal,
our 9th, vagus, our 10th, and the spinal accessory nerve.
You can actually see the groove leading to the internal jugular vein which is the sigmoid
sinus, but the venous sinuses – there are so many of them and this is a topic for another
time.
Looking in the midline you can see the largest of the foramina, the foramen magnum.
Lateral to those, there lies a foramen to specifically transmit the hypoglossal nerve,
and this is aptly called the hypoglossal canal.
As you can see, we have compartmentalised the skull and once you have done that, it’s
not so difficult.
It just requires a lot of pattern recognition and approaching the skull base with logical
mindset.
In conclusion, we have talked about the foramina of the skull base from the superior view and
subconsciously, we have learned the organisation of the cranial nerve and its numbering antero-posteriorly.
We know that the first cranial nerve goes through the cribriform plate – here,
and CNII, which is our optic nerve goes through the optic canal.
CNIII, IV, V¬1, VI goes through the SOF which is not visible from this view.
V2 goes through the foramen rotundum, V3 goes through the foramen ovale.
CNVII and CNVIII goes through the internal acoustic meatus.
CNIX, CNX, and CNXI goes through the jugular foramen, which are big foramina on each side.
And finally the hypoglossal nerve goes through the hypoglossal canal.
And as you can see, it goes antero-posteriorly and hopefully this lecture helped you remember
the organisation a bit better.
Thank you for watching and see you soon.
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