Okay so this is a tutorial on the Circle of Willis.  The Circle of Willis provides the blood supply to the brain, and essentially what happens is that it connects two arterial sources together to form this arterial circle, which then supplies the brain with blood. 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.

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 the arterial Circle of Willis.  Just taking a look at the origin of these arteries, you can see the internal carotid artery arising from the common carotid artery.

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, you can see that here. And then they run up through the transverse foramen of the cervical vertebrae.  These two arterial sources anastomose to form this arterial circle at the base of the brain.

Looking at this inferior aspect of the brain, I'm just going to switch over to a diagram.  You can see here how the Circle of Willis is located at the base of the brain, around the optic chiasm, and the hypothalamus.  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.

The internal carotid artery divides to form the middle cerebral artery and the anterior cerebral artery.  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 around 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.

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 around anteriorly, you can see this fissure running down between each hemisphere, and this is called the interhemispheric fissure, or the medial longitudinal fissure, and the anterior cerebral artery runs in this fissure.

I've just removed the right hemisphere, and rotating the model round, you can see the anterior cerebral artery running along in this fissure.  Just bringing the model back around 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 carotid artery, you've got the posterior communicating artery, which connects the internal carotid artery to the posterior cerebral artery.

It connects the anterior circulation provided by the internal carotid artery, to the posterior circulation, which is provided by the vertebro-basilar system, which I'm going to talk about next.

I've just highlighted the posterior communicating arteries in orange, and you can see how they connect to the posterior cerebral arteries.

Another artery to mention, just while we're talking about the internal carotid artery, is the ophthalmic artery.  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 far we've looked at the first arterial source, the internal carotid artery.  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 vertebro-basilar system, so it's a part of the posterior circulation.

Before describing the vertebro-basilar system, I'd like to just show you the territories of the brain which the anterior, the middle, and the posterior cerebral arteries supply.  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.

The middle cerebral artery is represented in red and the posterior cerebral artery is represented by the yellow shading.  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 cerebral artery, and you can see that posteriorly and inferiorly, 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.

Anterior cerebral artery supplies medial surface, middle cerebral artery supplies the lateral surface mainly.  Just coming back to the 3D model, the second source of blood is the vertebro-basilar system, which provides the posterior circulation.  this system supplies the cerebellum, the brainstem and the posterior parts of the cerebral hemispheres.

The basilar artery, highlighted in yellow, is formed by the union of the two vertebral arteries on either side.  These two arteries join together at the level of the bottom of the pons.  The inferior aspect of the pons.  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.

The basilar artery terminates by splitting into the posterior cerebral arteries.  Just taking a look at some of the branches of the vertebro-basilar system, there are three main branches which supply the cerebellum.  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.

You’ve got superior cerebellar arteries, and you've got inferior cerebellar arteries. And the inferior cerebellar arteries you've got anterior and posterior inferior cerebellar arteries.  At the bottom of the basilar 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.

They come off down here.  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.  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.  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.

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 then 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.

We've looked at the anterior circulation originating from the internal carotid artery, and we've looked at the posterior circulation, originating from the vertebro-basilar 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.