okay so this is a tutorial on the Circle
of Willis
so the Circle of Willis provides the
blood supply to the brain and
essentially what happens is that it
connects to arterial sources together to
form this arterial circle which then
supplies the brain with blood so the two
arteries which are the source of the
Circle of Willis are the internal
carotid arteries and you've also got the
vertebral arteries as well so just
rotating the model around to the lateral
view you can see the vertebral arteries
here coming up to supply the posterior
aspect of the Circle of Willis and you
can see the internal carotid arteries
here running up to join into the
arterial Circle of Willis so just taking
a look at the origin of these arteries
you can see the internal carotid artery
arising from the common carotid artery
so the common carotid artery bifurcates
to the external carotid and the internal
carotid arteries and the vertebral
arteries arise from the first part of
the subclavian artery so you can see
that here and then they run up through
the transverse foramen of the cervical
vertebra so these two arterial sources
and master Mo's to form this arterial
circle at the base of the brain so
looking at this inferior aspect of the
brain I'm just going to switch over to a
diagram so you can see here how the
Circle of Willis is located at the base
of the brain around the optic chiasm and
the hypothalamus so coming back to this
model here I've highlighted the internal
carotid artery and if I rotate the model
you can see how it joins this circle at
the base of the brain so the internal
carotid artery divides to form the
middle cerebral artery and the anterior
cerebral artery
so I've highlighted these in orange on
both sides so you can see the division
into the middle cerebral artery and the
anterior cerebral artery and if I rotate
the model round to the lateral view you
can see that the middle cerebral artery
actually runs in this lateral sulcus so
this this groove here which I'm
outlining in green separating the
frontal and parietal lobes above from
the temporal lobe below is known as the
lateral sulcus or lateral fissure and
you can see the middle cerebral artery
running within this fissure so within
this fissure the middle cerebral artery
gives off some small branches to supply
the basal ganglia and these are called
the striate arteries and just rotating
the model round anteriorly you can see
this fissure running down between each
hemispheres and this is called the inter
hemispheric fissure or the medial
longitudinal fissure and the anterior
cerebral artery runs in this fissure so
I've just removed the right hemisphere
and rotating the model around you can
see the anterior cerebral artery running
along in this fissure so just bringing
the model background to the inferior
view you can see that there's this small
connection between the anterior cerebral
arteries and this is called the anterior
communicating artery so it's a very
short artery which connects these two
together and then coming backwards off
the internal quartered artery
you've got the posterior communicating
artery which connects the internal
carotid artery to the posterior cerebral
artery so it connects the anterior
circulation provided by the internal
carotid artery to the posterior
circulation which is provided by the
vertebrobasilar system which I'm going
to talk about next so I've just
highlighted the posterior communicating
artery in orange and you can see how
they connect to the posterior cerebral
arteries
so another artery dimension just while
we're talking about the internal carotid
artery is the ophthalmic artery so
rotating the model around you can see
that the internal carotid artery gives
off this branch which is known as the
ophthalmic artery and these arise
immediately after the carotid artery
passes through the cavernous sinus to
enter the cranial cavity so so far we've
looked at the first arterial source the
internal carotid artery so we've seen
how it divides to form the middle
cerebral and the anterior cerebral
arteries and it also sends off a
posterior communicating branch to join
with the posterior cerebral artery which
comes off the vertebrobasilar system so
it's a part of the posterior circulation
so before describing the vertebrobasilar
system I'd like to show you the
territories of the brain which the
anterior the middle and the posterior
cerebral artery supply so I've just
switched over to two diagrams of the
brain so the top diagram shows the
medial aspect of the brain and what it
shows is that the anterior cerebral
artery which is represented by the blue
shaded area supplies the medial and the
superior and lateral aspects of the
cerebral hemisphere so the middle
cerebral artery is represented in red
and the posterior cerebral artery is
represented by the yellow shading so if
you look at the bottom diagram this
shows the lateral aspect of the brain so
you can see that the lateral aspect of
the brain is provided mostly by the
middle that cerebral artery and you can
see that posteriorly and in fairly the
shading is yellow in both the top and
bottom diagrams so the posterior
cerebral artery supplies the occipital
lobes and the inferior parts of the
brain so anterior cerebral artery
supplies medial surface middle cerebral
artery supplies the lateral surface
mainly so just coming back to the 3d
model the second source of blood is the
vertebrae basalis system which provides
the pasta
so this system supplies the cerebellum
the brainstem and the posterior parts of
the cerebral hemispheres so the basilar
artery highlighted in yellow is formed
by the union of the two vertebral
arteries on either side so these two
arteries joined together at the level of
the bottom of the pons so the inferior
aspect of the pons so I've just added in
the brainstem here and what you can see
is the basilar artery running in the
midline in front of the pons and then
you can see when it reaches the midbrain
it splits into the posterior cerebral
arteries so the basilar artery
terminates by splitting into the
posterior cerebral arteries so just
taking a look at some of the branches of
the vertebrae basalis system there are
three main branches which supplied the
cerebellum so starting at the top you
can see that there are two branches
which come off either side of the
basilar artery just before it terminates
to divide into the posterior cerebral
arteries and these are called the
superior cerebellar arteries so you've
got superior cerebellar arteries and
you've got inferior cerebellar arteries
at the inferior cerebellar arteries
you've got anterior and posterior
inferior cerebellar arteries so at the
bottom of the basal artery you can see
the anterior inferior cerebellar
arteries and then you've got the
posterior inferior cerebellar arteries
which actually come off the vertebral
arteries so they come off down here so
as well as giving off the superior
cerebellar artery and the anterior
inferior cerebellar artery the basilar
artery also gives off several small
pontine arteries which i've just drawn
on there so we've seen that the
vertebral arteries have the posterior
inferior cerebellar artery coming off it
but it's also got two other branches so
it's got the anterior spinal artery so
I've just drawn that on here and you can
see how it comes off the vertebral
arteries to unite and form this artery
which runs down the anterior median
fissure of the spinal cord and similarly
you've got a posterior spinal artery
which runs down the posterior aspect of
the spinal cord
so you've got three sort of branches
coming from the basilar artery the
superior cerebellar artery the anterior
inferior cerebellar artery and the
pontine arteries and you've got three
branches which come off the vertebral
arteries so the posterior inferior
cerebellar artery and then you've got
the anterior and posterior spinal
arteries so we've looked at the anterior
circulation originating from the
internal carotid artery and we've looked
at the posterior circulation originating
from the vertebral basalis system and
you can see that there's this
communication between both circulations
via the posterior communicating artery
which I've highlighted in green and this
artery connects the internal carotid to
the posterior cerebral artery which I've
outlined in yellow so that's the
arterial Circle of Willis