alright let's discuss the cranial base and specifically we're going to look at
a superior view of the cranial base so what had to be removed in order for us to be able to see
this your skullcap or your calvaria so those important bones that are protecting the brain
we remove those superiorly and now we're looking inside where the brain would sit typically and
as you can see it's in a very characteristic stair-step appearance you're going to have the
anterior cranial fossa which is the highest the middle cranial fossa and then the deepest and
the most the the largest area is going to be the posterior cranial fossa because not only is the
occipital lobe of the cerebrum sitting there but also the cerebellum so we'll discuss each one in
order and then we'll talk about the pertinent cranial nerves that are actually exiting this
region so let's start with the anterior cranial fossa this is going to extend from the squama
of your frontal bone all the way to the posterior portion of the lesser wing of the sphenoid so you
can see very wing like in appearance of the sphenoid bone and the sphenoid is this large
complex right here we'll discuss a lot more in terms of the foramina here but it's also going
to be this posterior border now your frontal lobe of the cerebrum sits right in this region
and the frontal lobe when you're talking about Homo sapiens very well-developed in comparison
to other mammals so that's why we have such a large area right here it's also why we have such
a substantial forehead in comparison to other mammals so very well developed but in terms of
the foramina in this region not quite as complex so where we're going to discuss right now is right
here in the midline you can see this this place right here referred to as the cribriform plate
and within that cribriform plate you're going to have these tiny little dots you're going to
have numerous foramina cribriform foramina where olfactory axons are actually coalescing to form
the olfactory nerves so think olfactory what does that stand for olfaction which has to do
with smell so this is your cranial nerve 1 which is going to be right in this region right here now
right in the middle of that you had the projection referred to as the crista galli and this is going
to be the anterior connection of the falx cerebri so that very dominant dural partition right in
the middle of the two cerebral hemispheres so that's going to be the anterior attachment of
that and that's all we really have in terms of this anterior cranial fossa the more complex
region is the middle cranial fossa so intermediate obviously it's going to extend from the posterior
border of this lesser wing of the sphenoid to the apex of the petrous portion of the temporal
bone now the petrous portion is probably the most dominant thing when you're looking at
this view it means rock-like or pyramid shaped in this region and we'll talk a little a lot
more in detail about what's actually in there but important in terms of your middle and inner
ear so let's start more anteriorly and we'll move posterior for the middle cranial fossa now this
one is particularly difficult to see but right underneath this lesser wing of the sphenoid is
your superior orbital fissure and we'll actually see this when you're looking into the orbit as
well so this large portion right here so this is the connection between this the internal portion
of the skull to the orbit so what type of nerves do you think are heading through there nerves that
are going to play a role in terms of eye movement so those extra ocular eye muscles the nerves that
innervate that so that would be oculomotor which is cranial nerve three trochlear cranial
nerve four and abducens cranial nerve six so all those three nerves are going to innervate those
extra ocular eye muscles additionally also going through this region is what the what's referred to
as the first division of the trigeminal nerve so trigeminal nerve is cranial nerve five it is the
largest of your cranial nerves it's so large that it actually divides into three divisions that have
their own names so this is the ophthalmic division of the trigeminal nerve so play a role in terms of
sensory innervation of the sinuses right underneath it you're going to have this very round foramen
referred to as foramen rotundum this is the second where the second division of trigeminal nerve
exits so the maxillary division of the trigeminal nerve so important in terms of innervating those
front or top teeth your maxillary teeth and then lastly you're gonna have your foramen ovale right
in this region and that's the third division of your trigeminal nerve so important in terms of the
muscles of mastication so efferently providing innervation to those muscles of mastication now
let's talk a little bit more about this petrous portion of the temporal bone so very hard very
prominent in this region and I think this is one of the neatest areas in the whole whole
skeleton because this is the only area where you have bone within bone so if I were to crack open
this region right here you would have three of your tiniest bones which are your ear ossicles
so your malleus your incus and your smallest bone your stapes those are all going to be located
in the middle ear and that will lead into the inner ear so very important in terms of hearing as
well as equilibrium so that is the middle cranial fossa in a nutshell and let's focus lastly on this
posterior cranial fossa and as we discuss this is going to be the deepest of your cranial fossa you
have your occipital lobe of the cerebrum sitting in here as well as the cerebellum now right in
this region on the opposite side of the petrous portion you have your internal acoustic meatus
you have your facial nerve or cranial nerve seven which is going to enter through here and
it'll eventually exit the skull facial nerve is going to come up over and over again throughout
our series very important in terms of innervating those muscles of facial expression so allowing us to
smile or to wink our eyes etc also cranial nerve eight is going to enter this region but it's not
actually ever going to exit the skull cranial nerve eight is referred to as vestibulocochlear
and as it as its name would suggest it plays a role in term of hearing as well as equilibrium
so very important makes sense that it's going into this region and innervating that okay continuing
our discussion probably another one of the most difficult of this to actually kind of show is your
jugular foramen and this is my favorite foramen of the body there's a lot going on here there's a lot
exiting the skull in this region you're going to have cranial nerves nine glossopharyngeal cranial
nerve 10 vagus and cranial nerve 11 accessory all exiting the skull through here so you have
a lot in terms of cranial nerves but importantly those venous sinuses those dural venous sinuses
that are draining the brain are going to coalesce right here to form the internal jugular vein so
if you've ever heard of cutting your jugulars typically if you're either talking about the
external jugular or the deeper internal jugular so the internal jugular is going to begin on the
other end of this region so draining the entirety of the brain lastly and probably the most dominant
thing in this region is the foramen magnum this is the second largest foramen in the body so even
though it's called foramen magnum it's actually not the largest this is where the brainstem becomes
the spinal cord so on the inferior end of the foramen magnum now we're talking spinal cord
your accessory nerve so cranial nerve 11 that we just discussed actually ascends through the
foramen magnum and then it will go back through and descend through the jugular foramen in order
to innervate your sternocleidomastoid muscles as well as your trapezius muscle