(4.15)

The arterial circle provides connections between the right and left sides, and also connects the vertebral and internal carotid systems. It's more of a hexagon than a circle.

Its component parts, from front to back, are the anterior communicating artery, the anterior cerebral arteries, the internal carotids, the posterior communicating arteries, and the posterior cerebral arteries. The arrangement is often somewhat asymmetrical: here, the left posterior communicating artery is very small.

The vessels we're looking at lie in the confined space between the floor of the cranial cavity and the underside of the brain. To see how they're related to the brain, we'll look at a brain that's been removed from the body together with its arteries. The arteries have been filled with red latex.

Over this area, the arachnoid layer and the many small vessels in it have been removed, so that we can see the major arteries. Here's the optic chiasm, here beneath it are the divided ends of the internal carotid arteries.

Here's the the anterior cerebral artery, passing around the optic chiasm, which we'll pull downwards. Here's the anterior communicating artery. The two anterior cerebral arteries turn upwards to enter the longitudinal cerebral fissure. We'll follow them shortly.

The internal carotid, which we'll go back to, gives off the posterior communicating artery, then continues, to become the middle cerebral artery. The middle cerebral artery enters the lateral cerebral fissure, between the frontal and temporal lobes of the cerebral hemisphere.

Coming from below, here are the two vertebral arteries joining to form the basilar artery, which is quite off-center in this specimen. Here are three of the possible four inferior cerebellar arteries, here are the two superior cerebellar arteries. Here's the division of the basilar, into the two posterior cerebral arteries.

To follow the course of the anterior, middle and posterior cerebral arteies, we'll divide the brain in the midline and look at just one cerebral hemisphere.

Each anterior cerebral artery runs upwards and then backwards close to the corpus callosum. It gives off branches which supply this area on the medial aspect of the cerebral hemisphere, and which then reach over the superior margin of the hemisphere, to supply this area on the lateral aspect.

Next we'll follow the middle cerebral artery. Here it is again, running in the depths of the lateral cerebral fissure. The middle cerebral artery gives off branches which emerge along the length of the lateral cerebral fissure to supply this area on the lateral aspect of the cerebral hemisphere.

Lastly we'll follow the posterior cerebral artery. It runs laterally just above this nerve, the oculomotor, then runs backward, passing around the cerebral peduncle. To follow it we'll again look at the cerebral hemisphere by itself.

Here's the posterior cerebral artery. It winds around between the cerebral peduncle, which has been divided here, and the most medial part of the temporal lobe.

The posterior cerebral artery gives off branches which supply this area on the medial aspect and underside of the hemisphere, and this area on the lateral aspect.