11 432 · 11 Blood Supply and Vascular Disorders of the Central Nervous System

The thalamogeniculate artery arises from the posterior cerebral artery distal to the origin of the posterior communicating artery (Fig. 11.10). It supplies the lateral portion of the thalamus.

The medial and lateral posterior choroidal arteries also arise distal to the origin of the posterior communicating artery (Figs. 11.9­11.10). They supply the geniculate bodies, medial and posteromedial thalamic nuclei, and pulvinar. The medial posterior choroidal artery gives off branches to the midbrain and supplies the choroid plexus of the third ventricle. The lateral posterior choroidal artery supplies the choroid plexus of the lateral ventricle and has an anastomotic connection with the anterior choroidal artery.

Cortical branches of the posterior cerebral artery. The territories of the posterior cerebral artery and middle cerebral artery vary widely in extent. In some cases, the posterior cerebral artery territory is delimited by the sylvian fissure; in others, the middle cerebral artery supplies the entire convexity of the brain all the way back to the occipital pole. The visual cortex of the calcarine sulcus is always supplied by the posterior cerebral artery. The optic radiation, however, is often supplied by the middle cerebral artery, so that homonymous hemianopsia does not always imply an infarct in the territory of the posterior cerebral artery. The posterior cerebral artery supplies not only the occipital lobe but also the medial temporal lobe through its temporal branches.

Collateral Circulation in the Brain

External-to-Internal Collateralization

When the internal carotid artery is stenotic, blood is diverted from branches of the external carotid artery into the internal carotid artery distal to the stenosis, enabling continued perfusion of the brain. The facial or superficial temporal artery, for example, can form an anastomotic connection with the ophthalmic artery by way of the angular artery; retrograde flow in the ophthalmic artery then takes the blood back into the carotid siphon (Fig. 11.11). Collaterals to the ophthalmic artery can also be fed by the buccal artery. Further external-to-in- ternal anastomotic connections exist between the ascending pharyngeal artery and meningeal branches of the ICA. These arteries, usually too small to be seen by angiography, are known collectively as the inferolateral trunk.

External-Carotid-to-Vertebral Collateralization

The branches of the external carotid artery and vertebral artery that supply the cervical and nuchal muscles are anastomotically connected at multiple points.

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Circle of willis

Superficial temporal a.

Occipital a.

Internal carotid a.

Vertebral a.

Arteries of the Brain · 433 11

Leptomeningeal branches and anastomoses

Ophthalmic a.

Angular a.

Facial a.

External carotid a.

Common carotid a.

Fig. 11.11 Anastomoses of the arteries of the brain. The following collateral pathways are shown:

Collaterals from the external to the internal carotid circulation: 1, external carotid artery−facial artery−angular artery−internal carotid artery; 2, external carotid artery−superficial temporal artery−angular artery−internal carotid artery. 3, Collaterals from the external to the vertebral circulation: external carotid artery−occipital artery−vertebral artery. 4, Circle of Willis. 5, Leptomeningeal collaterals between the anterior, middle, and posterior cerebral arteries. After Poeck K and Hacke W: Neurologie, 11th ed., Springer, Berlin/Heidelberg, 2001.

The most important branch of the external carotid artery in this respect is the occipital artery. Collaterals can form in either direction (Fig. 11.11): proximal occlusion of the vertebral artery can be compensated by blood from nuchal branches of the occipital artery, while occlusion of the common carotid artery

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11 434 · 11 Blood Supply and Vascular Disorders of the Central Nervous System

		Anterior communi-
		cating a.
		Anterior cerebral a.
	Olfactory tract	Superior chiasmatic a.
	Ophthalmic a.	Perforated substance
	Internal carotid a.	Medial and lateral
	Anterior cerebral a.	striate branches
		Recurrent a.
	Middle cerebral a.
		(of Heubner)
	Infundibulum	Superior
		hypophyseal a.
	Tuber cinereum	Supraoptic and
	Posterior communi-	paraventricular a.
		Anterior choroidal a.
	cating a.
	Mamillary body	Arteries to the
		tuber cinereum
	Oculomotor n.	Mamillary aa.
		Posterior thalamo-
		perforating a.
		Posterior cerebral a.
		Posterior perforating
		branches
		Superior cerebellar a.
		Basilar a.
	Fig. 11.12 Circle of Willis

or proximal occlusion of the internal carotid artery can be compensated by blood entering the anterior circulation from the muscular branches of the vertebral artery by way of the occipital artery. As another example, if a proximal occlusion of the common carotid artery has cut off both the internal and the external cerebral arteries from the circulation, then blood from the vertebral artery can flow in the external carotid artery in retrograde fashion down to the carotid bifurcation, and then up again in the internal carotid artery, restoring perfusion in the ICA territory.

Arterial Circle of Willis

Thecerebralarteriesareconnectedtoeach otherthrough a wreathlikearrangement of blood vessels at the base of the brain known as the circle of Willis (after ThomasWillis,anEnglishanatomistoftheseventeenthcentury).Thisinterconnection enables continued perfusion of brain tissue even if one of the great ves-

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Veins of the Brain · 435 11

selsisstenoticoroccluded.Thecircleitselfconsistsofsegments of the great vessels and the so-called communicating arteries linking them to one another. Traveling around one side of the circle from anterior to posterior, we find the anteriorcommunicatingartery,theproximal(A1)segmentoftheanteriorcerebral artery,thedistalsegmentoftheinternalcarotidartery,theposteriorcommunicating artery, the proximal (P1) segment of the posterior cerebral artery, and the basilar tip (Fig. 11.12). Decreased blood flow in a great vessel due to slowly progressive stenosis below the circle of Willis can usually be compensated by increased collateral flow around the circle, so that hemodynamic infarction will not occur (see below). There are, however, frequent anatomical variants of the circle of Willis in which one or more of its constituent arterial segments may be hypoplastic or absent. The unlucky combination of a stenotic great vessel with an anatomical variant of the circle of Willis preventing adequate collateral flow can result in hemodynamic infarction (p. 445 and Fig. 11.21).

Callosal Anastomoses

The anterior and posterior cerebral circulations are anastomotically connected through the callosal arteries (Fig. 11.1). Thus, when the anterior cerebral artery is occluded, blood from the posterior cerebral artery may continue to supply the central region.

Leptomeningeal Anastomoses

Furthermore, the branches of the anterior, posterior, and middle cerebral arteries are anastomotically linked to each other through the arteries of the pia mater and arachnoid (Fig. 11.11). There are also leptomeningeal anastomoses linking the branches of the three main cerebellar arteries.

Veins of the Brain

Superficial and Deep Veins of the Brain

The veins of the brain, unlike those of the rest of the body, do not run together with its arteries. The territories of the cerebral arteries do not coincide with the drainage areas of the cerebral veins. Venous blood from the brain parenchyma crosses the subarachnoid and subdural spaces in short cortical veins whose anatomy is relatively invariable: these include the superior anastomotic vein (of Trolard), the dorsal superior cerebral vein, the superficial middle cerebral vein, and the inferior anastomotic vein (of Labbé) on the lateral surface of the temporal lobe (Fig. 11.13).

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11 436 · 11 Blood Supply and Vascular Disorders of the Central Nervous System

Dorsal superior cerebral vv.

Inferior anasto-				Inferior
motic v.				cerebral v.
(of Labbé)

Fig. 11.14 Veins of the brain, medial view

Superior sagittal sinus

Inferior sagittal sinus

Internal occipital v.

Great cerebral v. (of Galen)

Straight sinus

Transverse sinus (truncated)

Occipital sinus (variant)

Superior anastomotic v. (of Trolard)

Superior sagittal sinus

Superficial middle cerebral v.

Fig. 11.13

Veins of the brain, lateral view

Medial superior cerebral vv.

Thalamostriate v.

Anterior cerebral v.

Vein of the septum pellucidum

Internal cerebral v.

Basal v. (of Rosenthal)

Thalamostriate v.

Internal cerebral v.

Basal v. (of Rosenthal)

Fig. 11.15 Veins of the interior of the brain and their territories in coronal section

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