Feeding the Brain: Four Arteries and a Safety Ring
The brain is three per cent of the body's weight and takes fifteen per cent of its blood, and it cannot store a single second of oxygen. Interrupt its supply and consciousness fails in seconds and the tissue starts to die in minutes. Yet the whole of that priceless organ is fed by only four arteries — two climbing up the front of the neck and two hidden in the bones of the spine — and they all pour into one small ring of vessels at the base of the brain. That ring is the reason a person can have one of the four blocked, sometimes for years, and never know it: the other three quietly cover for the missing one. It is also, at its junctions, the commonest site in the body for an artery to balloon and burst. To understand a stroke, a nosebleed that will not stop, a warning shadow across one eye, or a thunderclap headache, you have to know these vessels and the ring they build.
A 68-year-old man is having his carotids listened to at a routine check. The doctor presses the bell of the stethoscope lightly against the side of his neck, just below the angle of the jaw, and asks him to hold his breath. Over the pulse there is a soft, blowing murmur that keeps time with the heartbeat — a bruit. It is the sound of blood forcing its way past a narrowing where the common carotid splits, its internal branch furred with plaque. The man feels nothing; his brain is fully supplied, because the ring at its base is drawing extra flow from the other three arteries to make up the shortfall. But that plaque is a launch site. A fragment can break loose, ride the internal carotid up into the eye or the brain, and lodge — and then, without warning, a curtain will fall across one eye for a few minutes, or an arm and half a face will go weak. The whole of that story — the silence now, the catastrophe later — is written in the anatomy of four arteries and the ring they feed.
The subclavian artery and everything that springs from it
Before the arteries reach the head, they branch in the root of the neck — and one of those branches feeds half the brain. The two subclavian arteries begin differently on the two sides: the RIGHT arises from the BRACHIOCEPHALIC TRUNK, while the LEFT springs directly from the arch of the aorta, an asymmetry inherited from the great vessels described in the aorta and great vessels. Each arches over the first rib to become the axillary artery, and along the way gives a family of branches. The most important by far is the VERTEBRAL ARTERY, the first branch, which climbs almost vertically and threads through the TRANSVERSE FORAMINA of the sixth cervical vertebra up to the first (C6 to C1) before curving over the atlas and passing through the FORAMEN MAGNUM to enter the skull — it is one of the four brain arteries, hidden in the spine the whole way up. The others supply the wall and root of the neck: the INTERNAL THORACIC artery descending behind the costal cartilages; the THYROCERVICAL TRUNK, a short stump that fans into the INFERIOR THYROID artery (whose intimate relation to the recurrent laryngeal nerve makes it the surgeon's landmark in the thyroid gland), the SUPRASCAPULAR and the TRANSVERSE CERVICAL arteries; the COSTOCERVICAL TRUNK to the upper posterior chest wall; and a variable DORSAL SCAPULAR artery to the muscles of the back. Only one of all these — the vertebral — is bound for the brain.
Two systems, front and back
The brain is fed from two directions at once, and the two never quite meet until the very base of the brain. There are exactly two arterial systems to the brain, and it is worth fixing them apart before naming a single branch. The ANTERIOR circulation comes from the two INTERNAL CAROTID arteries. Each internal carotid, having given off nothing at all in the neck, enters the skull, gives the OPHTHALMIC artery to the eye, then the POSTERIOR COMMUNICATING and the ANTERIOR CHOROIDAL arteries, and finally splits into its two terminal branches — the ANTERIOR CEREBRAL and the MIDDLE CEREBRAL arteries. The POSTERIOR circulation comes from the two VERTEBRAL arteries, which, having entered the foramen magnum, join at the lower border of the pons to form a single midline vessel, the BASILAR ARTERY. The basilar runs up the front of the brainstem giving the POSTERIOR INFERIOR CEREBELLAR (PICA, actually the last big branch of each vertebral), the ANTERIOR INFERIOR CEREBELLAR, the PONTINE branches and the SUPERIOR CEREBELLAR arteries, before ending by dividing into the two POSTERIOR CEREBRAL arteries. So the front of the brain is carotid territory and the back is vertebrobasilar territory — two supplies, meeting only in the ring that joins them.
Think of a critical hospital fed by four separate power lines — two coming in from the front of the city, two from behind. Between them, right at the door of the building, the engineers have wired a single closed loop, a ring bus, into which all four lines feed and from which every ward draws. On a normal day each line supplies roughly its own quarter of the building. But the ring means the quarters are not sealed off from one another: cut any one incoming line and current simply runs the other way around the loop to fill the gap, and the lights in that quarter never so much as flicker. This is exactly the arrangement at the base of the brain. The four arteries do not each supply a private, walled-off territory with no way in from the neighbours; they empty into a shared ring — the circle of Willis — from which the cerebral arteries set out. That single design decision is why a slowly narrowing carotid can be silent for years, and why the same ring, overworked and turbulent at its joints, is where arteries fail.
The circle of Willis — the ring that lets one artery cover for another
The CIRCLE OF WILLIS is a ring of arteries lying on the base of the brain around the pituitary stalk and the optic chiasm, and it is the single most important anastomosis in the head because it joins the two carotid systems to each other and to the vertebrobasilar system behind. Trace it once and it never leaves you. At the back sit the two POSTERIOR CEREBRAL arteries, the terminal branches of the basilar. They are tied forward on each side to the internal carotid by a POSTERIOR COMMUNICATING artery. The internal carotids continue as the two ANTERIOR CEREBRAL arteries running forward and medially, and these two are joined across the midline by a single, short ANTERIOR COMMUNICATING artery. So the ring is: two anterior cerebrals bridged in front by one anterior communicating; two posterior cerebrals behind; and the two posterior communicating arteries closing the sides. Through it, blood from either carotid or from the basilar can, in principle, reach any part of the brain — a COLLATERAL pathway that can keep tissue alive when one feeding vessel blocks slowly. The catch is that the ring is anatomically COMPLETE and functional in only a minority of people; in most, one or more communicating vessels is thread-thin or absent, which is why the same carotid occlusion is silent in one patient and devastating in another. And the ring exacts a price: its junctions — above all where the anterior communicating meets the anterior cerebrals, and where the posterior communicating leaves the carotid — are the classic sites of BERRY (saccular) ANEURYSMS, thin-walled outpouchings that can rupture into the subarachnoid space.
The circle of Willis is a bet the body places before it is needed. In someone whose ring is complete and generous, a carotid can silt up over years and the brain simply reroutes — the patient may first learn of a totally occluded internal carotid as an incidental finding, never having had a symptom, because flow crossed the anterior communicating from the other side and up the posterior communicating from the back. In someone whose ring is incomplete — a hypoplastic posterior communicating, an absent anterior communicating — that same occlusion, or even a brisk drop in blood pressure, strips a whole territory of its supply. This is why two patients with the identical angiogram of a blocked artery can have utterly different fates, and why a surgeon planning to clamp a carotid will first test whether the opposite side and the ring can carry the brain alone. The anatomy you are born with, invisible and asymptomatic your whole life, decides how much a stroke will cost you.
- Four arteries feed the brain: the two INTERNAL CAROTIDS (anterior circulation) and the two VERTEBRALS (posterior circulation) — the vertebrals join to form the single BASILAR artery.
- The SUBCLAVIAN is right from the brachiocephalic trunk, left from the aortic arch. Its branches: VERTEBRAL (first, up the transverse foramina C6–C1 to the foramen magnum), internal thoracic, THYROCERVICAL trunk (inferior thyroid, suprascapular, transverse cervical), costocervical trunk and dorsal scapular.
- The INTERNAL CAROTID gives NO branch in the neck; inside the skull it gives ophthalmic, posterior communicating and anterior choroidal, then divides into the ANTERIOR and MIDDLE CEREBRAL arteries.
- The BASILAR gives PICA (from the vertebral), AICA, pontine branches and the superior cerebellar arteries, then ends as the two POSTERIOR CEREBRAL arteries.
- The CIRCLE OF WILLIS = 2 internal carotids + 2 posterior cerebrals (from the basilar), joined in front by the 2 anterior cerebrals and the single ANTERIOR COMMUNICATING, and on each side by the POSTERIOR COMMUNICATING — a collateral ring, anatomically complete in only a minority, and the classic site of berry aneurysms at its junctions.
Which artery owns which piece of brain
Name the territory and you can read the stroke backwards from the deficit at the bedside. Each cerebral artery irrigates a defined slab of cortex, and the map is worth memorising because it turns a set of symptoms into a location. The ANTERIOR CEREBRAL artery sweeps over the MEDIAL surface of the frontal and parietal lobes — including the strip of motor and sensory cortex that serves the LEG and foot. The MIDDLE CEREBRAL artery, the direct continuation of the internal carotid and the vessel an embolus most often enters, supplies the vast LATERAL surface of the hemisphere: the motor and sensory cortex for the FACE and ARM, and — on the DOMINANT hemisphere, the left in most people — the language areas for SPEECH. The POSTERIOR CEREBRAL artery, from the basilar, wraps around to the OCCIPITAL lobe and the VISUAL cortex. From this alone the stroke syndromes follow. A middle cerebral occlusion gives weakness and numbness of the CONTRALATERAL face and arm (with the leg relatively spared) and, if it is the dominant side, APHASIA. An anterior cerebral occlusion does the opposite emphasis: weakness of the contralateral LEG. A posterior cerebral occlusion takes the opposite half of the VISUAL FIELD. The face-and-arm versus leg split is not arbitrary — it is simply the shape of the motor homunculus laid over the shape of the arterial territories.
A 71-year-old woman describes it precisely: a grey shade dropped down over her right eye "like a blind coming down", lasted about five minutes, then lifted completely, and she saw normally again. She thought nothing of it until, two days later, her left hand and the left side of her mouth went clumsy for half an hour and recovered. The eye episode is AMAUROSIS FUGAX — a fleeting monocular blindness from a tiny embolus passing up the internal carotid into the ophthalmic artery and then washing through. The limb episode is a hemispheric TRANSIENT ISCHAEMIC ATTACK (TIA), an embolus into the middle cerebral territory that lysed before it killed tissue. Both point to the SAME source: an ulcerated plaque at the carotid bifurcation on the RIGHT (the eye and the motor cortex it warned about are supplied from the same internal carotid). These are not trivial events; they are the brain's warning shots, and the risk of a completed, permanent stroke in the days that follow is high. The right response is urgent — imaging the carotid, antiplatelet therapy, and, for a tightly stenosed symptomatic artery, carotid endarterectomy — because the next embolus may not dissolve.
The external carotid — the face's own supply
While the internal carotid climbs to the brain giving nothing to the neck, its sibling the EXTERNAL CAROTID does the opposite: it stays outside the skull and hands out EIGHT branches that supply the face, scalp, upper neck viscera and the dura, a story told in full in the carotid and jugular. In brief, from below upward they are the superior thyroid, ascending pharyngeal, lingual, facial, occipital, posterior auricular, and then its two terminal branches, the MAXILLARY (whose middle meningeal branch supplies the dura and is the artery torn in an extradural haemorrhage) and the SUPERFICIAL TEMPORAL, which you can feel pulsating in front of the ear. These vessels, and the scalp and face they feed, anastomose with unusual freedom — across the MIDLINE with their fellows of the other side, and between the external and internal carotid systems (the facial and superficial temporal arteries meet the ophthalmic artery's branches around the eye). The consequences are described in the scalp and face: a scalp laceration bleeds ferociously and from both edges because it is fed from all directions, but the same rich cross-supply is why facial and scalp flaps survive on a single pedicle and why the face rarely suffers ischaemic loss.
How the blood gets back — the venous drainage
The return route is not a mirror of the arteries, and one small connection in it can turn a pimple into meningitis. Blood from the brain itself does not leave in veins that accompany the arteries; it collects into the DURAL VENOUS SINUSES — endothelium-lined channels running between the two layers of the dura, described with the coverings of the brain in the meninges and dural venous sinuses. These sinuses funnel, by way of the transverse and then the SIGMOID SINUS, down through the jugular foramen to become the INTERNAL JUGULAR VEIN, the principal drain of the head, which runs down the neck within the carotid sheath and empties into the brachiocephalic vein. The FACE and SCALP drain more superficially: through the FACIAL VEIN, the RETROMANDIBULAR VEIN (formed within the parotid gland) and the EXTERNAL JUGULAR VEIN over the sternocleidomastoid. The clinically vital detail is a communication: the veins of the face are VALVELESS, and the facial vein connects, through the superior ophthalmic vein in the orbit, with the CAVERNOUS SINUS inside the skull. This makes the triangle from the upper lip and nose to the bridge of the nose the DANGER AREA of the face — infection there can, rarely, spread backwards against the normal flow into the cavernous sinus and cause a septic cavernous sinus thrombosis. Finally, running the length of the vertebral column is the valveless VERTEBRAL VENOUS PLEXUS, an alternative low-pressure drainage route that connects the pelvic and thoracic veins with the veins around the brain.
- CEREBRAL TERRITORIES: anterior cerebral = medial hemisphere and the LEG area; middle cerebral = lateral hemisphere, FACE and ARM, and SPEECH on the dominant side; posterior cerebral = occipital/VISUAL cortex.
- STROKE by territory: middle cerebral → contralateral face-and-arm weakness (± aphasia if dominant); anterior cerebral → contralateral leg weakness; posterior cerebral → contralateral visual field loss.
- The EXTERNAL CAROTID has 8 branches (superior thyroid, ascending pharyngeal, lingual, facial, occipital, posterior auricular, maxillary, superficial temporal) supplying face, scalp, neck viscera and — via the middle meningeal branch of the maxillary — the dura.
- VENOUS DRAINAGE: brain → dural venous sinuses → sigmoid sinus → INTERNAL JUGULAR VEIN; face and scalp → facial, retromandibular and external jugular veins. The valveless facial vein connects to the CAVERNOUS SINUS — the facial DANGER AREA.
- Scalp and face arteries ANASTOMOSE freely across the midline and between the internal and external carotid systems; the vertebral venous plexus is a valveless alternative venous route along the spine.
- GIANT CELL ARTERITIS inflames the SUPERFICIAL TEMPORAL artery — a tender, pulseless, cord-like scalp artery with headache and jaw claudication, an ophthalmic emergency because it can blind through the ophthalmic artery.
- Thinking the internal carotid gives branches in the neck. It gives NONE — it is smooth all the way from the bifurcation to the skull. The external carotid is the one that branches. A branch arising in the neck therefore belongs to the external carotid, and this is a key surgical sign in identifying the two.
- Mislocalising the leg versus face-and-arm weakness. The ANTERIOR cerebral artery serves the LEG (it runs onto the medial surface where the leg homunculus sits); the MIDDLE cerebral serves the face and arm on the lateral surface. Reversing them reverses the artery you blame.
- Assuming the circle of Willis always protects. It is anatomically complete in only a minority; hypoplastic or absent communicating arteries are common, which is why an identical carotid occlusion is silent in one person and devastating in another. Never promise the ring will compensate.
A right-handed patient develops sudden weakness and numbness of the LEFT face and arm, with the left leg relatively spared, and cannot produce speech. Which artery is occluded, and on which side?
- The brain (3% of body weight, 15% of the blood, no oxygen store) is fed by FOUR arteries: the two internal carotids (anterior circulation) and the two vertebrals (posterior circulation, which join to form the single basilar artery). The vertebral is the first branch of the subclavian, climbing the transverse foramina C6–C1 to the foramen magnum.
- The CIRCLE OF WILLIS at the base of the brain joins the two carotids and the basilar into one ring (2 anterior cerebrals + 1 anterior communicating in front, 2 posterior cerebrals behind, 2 posterior communicating at the sides). It is a collateral safety ring — complete in only a minority — and its junctions are the classic site of berry aneurysms and subarachnoid haemorrhage.
- TERRITORIES and STROKE: anterior cerebral = leg; middle cerebral = face-and-arm plus speech on the dominant side; posterior cerebral = visual cortex. Hence contralateral face-and-arm weakness ± aphasia (MCA), contralateral leg weakness (ACA), or visual field loss (PCA); TIA and amaurosis fugax warn of carotid disease.
- The external carotid (8 branches) supplies the face, scalp and dura; drainage is brain → dural sinuses → sigmoid → internal jugular vein, with the valveless facial-vein connection to the cavernous sinus (the DANGER AREA) and the vertebral venous plexus. Watch too for vertebral dissection in neck manipulation and giant cell arteritis of the superficial temporal artery.
- Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students — Head and Neck: the subclavian and carotid arteries, the circle of Willis and the dural venous sinuses.
- Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy — Blood supply of the brain; the arterial circle; venous drainage of the head and neck.
- Netter FH. Atlas of Human Anatomy — Arteries and veins of the brain; arteries of the neck; the arterial circle of Willis.
- Snell RS. Clinical Anatomy by Regions — The blood supply of the brain and the cerebral arterial circle; stroke syndromes by territory.
- Last RJ. Last's Anatomy: Regional and Applied — The great vessels of the neck and the vertebral artery.
- TeachMeAnatomy — Arterial Supply to the Brain (Circle of Willis); The Subclavian Artery; Venous Drainage of the Head and Neck.
