Two cerebral circulation systems are described: the anterior or carotid and the posterior or vertebral-basilar.

Posterior or Vertebral-Basilar System

Vertebral Arteries (Fig. 16-1)

Identify the vertebral arteries as they travel along the anterior or lateral surfaces of the medulla, and join to form the basilar artery near the pontomedullary junction.

After entering the cranial cavity each vertebral artery gives rise to a posterior spinal artery which descends along the posterolateral aspect of the spinal cord. One to 2 cm before joining to form the basilar artery, the vertebral arteries give rise to their largest branches, the posterior inferior cerebellar arteries.

Observe the posterior inferior cerebellar arteries (PICA) as they curve around the medulla ventral to the roots of CN IX, X, and XI. Multiple penetrating vessels supplying the posterolateral medulla arise from PICA.

Immediately before the vertebral-basilar junction, anterior spinal arteries arise from both vertebral arteries and join almost immediately to form a single anterior spinal artery which runs along the anterior median fissure of the spinal cord.

Basilar Artery

Follow the basilar artery as it travels in the shallow median groove on the ventral surface of the pons to end at the midbrain where it divides into the posterior cerebral arteries. As the basilar artery travels along the pons it supplies multiple vessels that penetrate the pons as paramedian, short circumferential, and long circumferential arteries. Symmetrical large branches arising at about the middle of the basilar artery are the anterior inferior cerebellar arteries (AICA). Similar large, paired vessels arising just proximal to the termination of the basilar artery are the superior cerebellar arteries.

Posterior Cerebral Artery

Identify the posterior cerebral arteries (PCA) which begin at the basilar bifurcation. A short distance after arising the PCA anastomoses with the posterior communicating artery, thus connecting the anterior and posterior cerebral circulations (Figs. 16-1, 16-2). The PCA swings anterior to the oculomotor nerve, passes laterally along the surface of the cerebral crus to reach the dorsal surface of the free margin of the tentorium, and then proceeds posteriorly along the inferomedial surface of the temporal lobe. It supplies the medial and inferior surfaces of the temporal and occipital lobes. The PCA ends by forming the parieto-occipital and calcarine arteries found in the respective sulci. The calcarine artery supplies the primary visual area. The cortical branches of the PCA extend slightly onto the lateral surfaces of the temporal and occipital lobes where they anastomose with branches of the middle cerebral artery

Anterior or Carotid System

Identify the internal carotid artery (Fig. 16-1) on the ventral surface of the brain lateral to the optic chiasm. Its first two branches, the ophthalmic and superior hypophysial arteries, are usually destroyed in removal of the brain from the cranial cavity.

Posterior Communicating Artery

This artery leaves the internal carotid artery just before its terminal branching, passes posteriorly and joins the proximal portion of the posterior cerebral artery, thus connecting the anterior and posterior circulations.

Anterior Choroidal Artery

This usually arises from the internal carotid just proximal to its bifurcation. Sometimes, however, it arises from the middle cerebral artery, the posterior communicating artery, or from the bifurcation of the middle and anterior cerebral arteries. The anterior choroidal artery crosses the optic tract and passes toward the medial surface of the temporal lobe.

Anterior Cerebral Artery

The anterior cerebral artery is divided into proximal or precommunicating (A-1) and distal or postcommunicating (A-2) segments by the anterior communicating artery.

The A-1 segment begins at the carotid bifurcation and passes over the optic tract and chiasm to reach the anterior communicating artery. Along its course perforators, the anteromedial central or medial striate arteries, penetrate the anterior perforated substance.

Recurrent Artery (of Huebner) (Fig. 16-3)

This vessel is one of the medial striates and is conspicuous by its large size. It arises either from the distal part of the A-1 segment or the proximal part of the A-2 segment and courses laterally along the A-1 segment to join the lateral striate arteries as they enter the anterior perforated substance.

Anterior Communicating Artery

The anterior communicating artery joins the two anterior cerebral arteries. Anatomically, the anterior communicating artery is seldom a distinct vessel but more often constitutes a complex network or web of vessels.

Middle Cerebral Artery

The middle cerebral artery is the largest branch of the internal carotid and the cerebral artery most often occluded. It is divided into a proximal (M-1) segment and a distal (M-2) segment by the middle cerebral artery bifurcation.

M-1 Segment

Identify the proximal portion of the middle cerebral artery which is related to the lowest portion of the insula as the artery travels to the lateral or sylvian fissure. Ten to 15 penetrating vessels (lateral striate or lenticulostriate arteries) arise from this segment.

Clinically, these vessels are the most common site of spontaneous hypertensive hemorrhage in individuals afflicted with long-standing hypertension.

M-2 Segment

The bifurcation of the middle cerebral artery is located at the base of the insula and it forms the M-2 segment which consists of the superior and inferior trunks. These trunks travel deep in the lateral (sylvian) fissure along the insula. They eventually reach the lateral surface of the hemisphere and their branches supply the lateral aspects of the frontal, parietal, temporal, and occipital lobes.

The Central Branches of the Cerebral Arterial Circle

Identify the cerebral arterial circle (of Willis) (Fig. 16-2)and its branches that penetrate the ventral surface of the brain. These are called the central or ganglionic branches and are divided into four groups: anteromedial, anterolateral, posteromedial, and posterolateral (Fig. 16-3).

Anteromedial Arteries

These arise chiefly from the A-1 segment of the anterior cerebral artery although some may arise from the most proximal part of the A-2 segment, the anterior communicating artery, or the most terminal part of the internal carotid artery. Collectively they are referred to as the medial striate arteries and they enter the brain in the medial third of the anterior perforated substance. The largest and most lateral of these to enter the brain is the medial striate or recurrent artery of Huebner.

Anterolateral Arteries

These usually arise entirely from the M-1 segment of the middle cerebral artery although sometimes a few may come from the initial part of the anterior cerebral artery. They are frequently called the lateral striate or lenticulostriate arteries and they enter the brain in the lateral two-thirds of the anterior perforated substance

As mentioned previously these vessels are the most common sites of spontaneous hemorrhage in individuals with long-standing hypertension. For this reason one of them, not anatomically identifiable, is called the "artery of cerebral hemorrhage".

Posteromedial Arteries

These arise from the posterior communicating artery and the posterior cerebral artery proximal to where it anastomoses with the former. Those from the posterior cerebral artery are its thalamoperforating branches which enter the brain in the posterior perforated substance.

Posterolateral Arteries

These arise from the posterior cerebral artery distal to its anastomosis with the posterior communicating. They are also called the thalamogeniculate arteries.