(3.16)

Now that we've followed the internal carotid and vertebral arteries into the cranial cavity we'll see how they supply the brain. We'll also see the set of arterial connections known as the arterial circle, or circle of Willis.

So far we've seen the internal carotids entering up here, the vertebral arteries entering down here and joining to form the basilar artery. Now we'll complete the picture.

To name the vessels we're looking at, we'll start with the main branches of the internal carotid. The internal carotid gives off the anterior cerebral, and posterior communicating arteries, then continues with a different name: from here the vessel is called the middle cerebral artery.

The two anterior cerebral arteries curve towards each other above the chiasm, then pass upwards and forwards close together to enter the longitudinal cerebral fissure between the two cerebral hemispheres.

Just above the optic chiasm, the two anterior cerebral arteries are connected to each other by this very short anterior commumicating artery, which is part of the arterial circle.

The middle cerebral artery, which is the direct continuation of the internal carotid, curves laterally. It enters the lateral cerebral fissure between the frontal and temporal lobes. We'll follow it there shortly. The pale areas on this artery are patches of atheroma.

Now we'll go round to a view from behind, to see the vertebral and basilar arteries, and the vessels that arise from them. Here are the two vertebral arteries, joining together to form the basilar artery.

Down here four inferior cerebellar arteries usually arise, two posterior and two anterior. These are the posterior ones. In this specimen the anterior ones are represented by this one vessel. In addition the basilar artery gives off small branches to the pons, and this labyrinthine artery that supplies the inner ear.

Four branches arise from the top of the basilar artery, these two superior cerebellar arteries, and the two terminal branches of the basilar, the posterior cerebral arteries.

The posterior cerebral artery curves backwards and laterally above this nerve, the oculomotor. It curls around the cerebral peduncle. We'll look at its course in a few minutes. Just as it turns, the posterior cerebral artery is joined by this small artery that we've seen already: the posterior communicating artery.

The posterior communicating artery completes the arterial circle.