INTERNAL CAROTID ARTERY

The internal carotid arteries (Fig. 17.1) and their major branches (the internal carotid system or ‘anterior’ circulation) supply blood to the majority of the forebrain. Some parts of the occipital and temporal lobes are supplied by branches of the vertebrobasilar system (see Fig. 17.5).

Fig. 17.1 Internal carotid arteriograms. A, lateral projection B, Towne’s projection.

Fig. 17.5 The arteries supplying the left cerebral hemispheres. A, Lateral surface. B, Medial surface.

The internal carotid artery arises from the bifurcation of the common carotid artery, ascends in the neck and enters the carotid canal of the temporal bone. Its subsequent course is said to have petrous, cavernous and intracranial parts.

Intracranial part

After piercing the dura mater, the internal carotid artery turns back below the optic nerve to run between it and the oculomotor nerve. It reaches the anterior perforated substance at the medial end of the lateral fissure and terminates by dividing into the anterior and middle cerebral arteries.

Several preterminal vessels leave the cerebral portion of the internal carotid. The ophthalmic artery arises from the anterior part of the internal carotid as it leaves the cavernous sinus, often at the point of piercing the dura, and enters the orbit through the optic canal. The posterior communicating artery (Fig. 17.2, Fig. 17.3) runs back from the internal carotid above the oculomotor nerve, and anastomoses with the posterior cerebral artery (a terminal branch of the basilar artery), thereby contributing to the circulus arteriosus around the interpeduncular fossa. The posterior communicating artery is usually very small. However, sometimes it is so large that the posterior cerebral artery is supplied via the posterior communicating artery rather than from the basilar artery (‘fetal posterior communicating artery’); it is often larger on one side only. Small branches from its posterior half pierce the posterior perforated substance together with branches from the posterior cerebral artery. Collectively they supply the medial thalamic surface and the walls of the third ventricle. The anterior choroidal artery leaves the internal carotid near its posterior communicating branch and passes back above the medial part of the uncus. It crosses the optic tract to reach and supply the crus cerebri of the midbrain, then turns laterally, recrosses the optic tract, and gains the lateral side of the lateral geniculate body, which it supplies with several branches. It finally enters the inferior horn of the lateral ventricle via the choroidal fissure and ends in the choroid plexus. This small, but important, vessel also contributes to the blood supply of the globus pallidus, caudate nucleus, amygdala, hypothalamus, tuber cinereum, red nucleus, substantia nigra, posterior limb of the internal capsule, optic radiation, optic tract, hippocampus and the fimbria of the fornix.

Fig. 17.2 The arteries on the base of the brain. The anterior part of the left temporal lobe has been removed to display the initial course of the middle cerebral artery within the lateral fissure.

Fig. 17.3 The circulus arteriosus on the base of the brain showing the distribution of central (perforating or ganglionic) branches.

ANTERIOR CEREBRAL ARTERY

The anterior cerebral artery is the smaller of the two terminal branches of the internal carotid (Fig. 17.3).

Surgical nomenclature divides the vessel into three parts: A₁ – from the termination of the internal carotid artery to the junction with the anterior communicating artery; A₂ – from the junction with the anterior communicating artery to the origin of the callosomarginal artery; and A₃ – distal to the origin of the callosomarginal artery; this segment is also known as the pericallosal artery.

The anterior cerebral artery starts at the medial end of the stem of the lateral fissure. It passes anteromedially above the optic nerve to the great longitudinal fissure where it connects with its fellow by a short transverse anterior communicating artery. The anterior communicating artery is about 4 mm in length and may be double. It gives off numerous anteromedial central branches that supply the optic chiasma, lamina terminalis, hypothalamus, para-olfactory areas, anterior columns of the fornix and the cingulate gyrus.

The two anterior cerebral arteries travel together in the great longitudinal fissure. They pass around the curve of the genu of the corpus callosum and then along its upper surface to its posterior end, where they anastomose with posterior cerebral arteries (Fig. 17.4). They give off cortical and central branches.

Fig. 17.4 The major arteries supplying the medial, A, and lateral, B, aspects of the brain.

The cortical branches of the anterior cerebral artery are named according to their distribution. Two or three orbital branches ramify on the orbital surface of the frontal lobe and supply the olfactory cortex, gyrus rectus and medial orbital gyrus. Frontal branches supply the corpus callosum, cingulate gyrus, medial frontal gyrus and paracentral lobule. Parietal branches supply the precuneus, while the frontal and parietal branches both send twigs over the superomedial border of the hemisphere to supply a strip of territory on the superolateral surface (Fig. 17.5). Cortical branches of the anterior cerebral artery therefore supply the areas of the motor and somatosensory cortices that represent the lower limb.

Central branches of the anterior cerebral artery arise from its proximal portion and enter the anterior perforated substance (Fig. 17.3) and lamina terminalis. Collectively, they supply the rostrum of the corpus callosum, the septum pellucidum, the anterior part of the putamen, the head of the caudate nucleus and adjacent parts of the internal capsule. Immediately proximal or distal to its junction with the anterior communicating artery, the anterior cerebral artery gives rise to the medial striate artery which supplies the anterior part of the head of the caudate nucleus and adjacent regions of the putamen and internal capsule.

MIDDLE CEREBRAL ARTERY

The middle cerebral artery is the larger terminal branch of the internal carotid.

Surgical nomenclature divides the vessel into four parts: M₁ – from the termination of the internal carotid artery to the bi/trifurcation, this segment is also known as the sphenoidal; M₂ – the segment running in the lateral (Sylvian) fissure, also known as the insular; M₃ – coming out of the lateral fissure, also known as the opercular; and M₄ – cortical portions.

The middle cerebral artery runs at first in the lateral fissure, then posterosuperiorly on the insula, and divides into branches distributed to the insula and the adjacent lateral cerebral surface (Fig. 17.3, Fig. 17.4, Fig. 17.5). Like the anterior cerebral artery, it has cortical and central branches.

Cortical branches send orbital vessels to the inferior frontal gyrus and the lateral orbital surface of the frontal lobe. Frontal branches supply the precentral, middle and inferior frontal gyri. Two parietal branches are distributed to the postcentral gyrus, the lower part of the superior parietal lobule and the whole inferior parietal lobule. Two or three temporal branches supply the lateral surface of the temporal lobe. Cortical branches of the middle cerebral artery therefore supply the motor and somatosensory cortices that represent the whole of the body (other than the lower limb), the auditory area and the insula.

Small central branches of the middle cerebral artery, the lateral striate or lenticulostriate arteries, arise at its origin and enter the anterior perforated substance together with the medial striate artery. Lateral striate arteries ascend in the external capsule over the lower lateral aspect of the lentiform complex, then turn medially, traverse the lentiform complex and the internal capsule and extend as far as the caudate nucleus.

VERTEBRAL ARTERY

The vertebral arteries and their major branches (sometimes referred to as the ‘vertebrobasilar system’) essentially supply blood to the upper spinal cord, the brain stem and cerebellum and a significant but variable part of the posterior cerebral hemispheres (Fig. 17.6).

Fig. 17.6 Vertebral arteriograms. A, lateral projection; B, Towne’s projection.

The vertebral arteries are derived from the subclavian arteries (see Ch. 30). They ascend through the neck in the foramina transversaria of the upper six cervical vertebrae and enter the cranial cavity through the foramen magnum, close to the anterolateral aspect of the medulla (Fig. 17.2). They converge medially as they ascend the medulla and unite to form the midline basilar artery at approximately the level of the junction between the medulla and pons.

One or two meningeal branches arise from the vertebral artery near the foramen magnum and ramify between the bone and dura mater in the posterior cranial fossa. They supply bone, diploë and the falx cerebelli.

A small anterior spinal artery arises near the end of the vertebral artery, and descends anterior to the medulla oblongata to unite with its fellow from the opposite side at mid-medullary level. The single trunk then descends on the ventral midline of the spinal cord, and is reinforced sequentially by small spinal rami from the vertebral, ascending cervical, posterior intercostal and first lumbar arteries, which all enter the vertebral canal via intervertebral foramina. Branches from the anterior spinal arteries and the beginning of their common trunk are distributed to the medulla oblongata.

The largest branch of the vertebral artery is the posterior inferior cerebellar artery (Fig. 17.6A). It arises near the lower end of the olive and then ascends behind the roots of the glossopharyngeal and vagus nerves to reach the inferior border of the pons. Here it curves and descends along the inferolateral border of the fourth ventricle before it turns laterally into the cerebellar vallecula between the hemispheres, and divides into medial and lateral branches. The medial branch runs back between the cerebellar hemisphere and inferior vermis, and supplies both. The lateral branch supplies the inferior cerebellar surface as far as its lateral border and anastomoses with the anterior inferior and superior cerebellar arteries (from the basilar artery). The trunk of the posterior inferior cerebellar artery supplies the medulla oblongata dorsal to the olivary nucleus and lateral to the hypoglossal nucleus and its emerging nerve roots. It also supplies the choroid plexus of the fourth ventricle and sends a branch lateral to the cerebellar tonsil to supply the dentate nucleus. The posterior inferior cerebellar artery is sometimes absent.

A posterior spinal artery usually arises from the posterior inferior cerebellar artery, but may originate directly from the vertebral artery near the medulla oblongata. It passes posteriorly and descends as two branches which lie anterior and posterior to the dorsal roots of the spinal nerves. These are reinforced by spinal twigs from the vertebral, ascending cervical, posterior intercostal and first lumbar arteries, all of which reach the vertebral canal by the intervertebral foramina, and sustain the posterior spinal arteries to the lower spinal levels.

Minute medullary arteries arise from the vertebral artery and its branches and are distributed widely to the medulla oblongata.

BASILAR ARTERY

The basilar artery is a large median vessel formed by the union of the vertebral arteries at the mid-medullary level (Figs 17.2, 17.3, 17.6). It lies in the pontine cistern, and follows a shallow median groove on the ventral pontine surface, extending to the upper border of the pons. It ends by dividing into two posterior cerebral arteries at a variable level behind the dorsum sellae, usually in the interpeduncular cistern.

Numerous small pontine branches arise from the front and sides of the basilar artery along its course and supply the pons. The long and slender labyrinthine (internal auditory) artery has a variable origin. It usually arises from the anterior inferior cerebellar artery, but variations in its origin include the lower part of the basilar artery, the superior cerebellar artery or, occasionally, the posterior inferior cerebellar artery. The labyrinthine artery accompanies the facial and vestibulocochlear nerves into the internal acoustic meatus and is distributed to the internal ear.

The anterior inferior cerebellar artery (Fig. 17.3