Copyright photo : Par John A Beal, PhDDep’t. of Cellular Biology & Anatomy, Louisiana State University Health Sciences Center Shreveport http://www.healcentral.org/healapp/showMetadata?metadataId=40566 (Internet Archive of file description page), CC BY 2.5
It is ensured by a simple system :
• carotid arteries in front
• vertebral arteries back
2. The carotid system
It begins with the two primary carotid arteries originating from the aortic arch. This arch, in horizontal portion, gives three important collaterals: the right brachiocephalic artery trunk, either common or right primary carotid artery and right subclavian artery; the left primary carotid artery ascending vertically on the left flank of the visceral bundle of the neck; the left subclavian artery.
The two primitive carotids therefore ascend vertically and, at the hyoid bone, they divide into an internal carotid artery, ascending to the rock and giving no collateral in the neck and an external carotid artery giving very quickly collaterals, the first being the upper thyroid artery.
Only the internal carotid arteries will provide blood supply to most of the cerebral hemispheres.
The internal carotid artery reaches the petrous rock, from the temporal bone, leaves it, then reaches the body of the sphenoid. There it crosses the cavernous sinus, disposing of either side of the sphenoid body. It then describes the carotid siphon and leaves the cavernous sinus by its upper surface: the roof of the cavernous sinus. In the cavernous sinus, the internal carotid artery gives off collaterals, the first, which will essentially go to the pituitary gland.
The internal carotid artery then leaves the cavernous sinus to arrive inside the skull where it gives its collateral and terminal branches.
3. Vertebral arteries
They arise from their respective subclavian arteries and reach the transverse canal located in the lateral massifs of the cervical vertebrae, generally from C6. From there, the two vertebral arteries will travel through the canal to C1. They then reach the foramen magnum, crossing the meninge, and arrange themselves in front of the brainstem.
4. At the base of the skull
The two internal carotid arteries arrange themselves anteriorly, outside the anterior clinoid processes of the sphenoid.
The two vertebral arteries are anterior and laterally to the foramen magnum. They go upwards and inwards and anastomose at the most anterior point of the bulbo-protuberantial or bulbo-pontic sulcus to form the basilar artery. This rises behind the clivus, in front of the annular protuberance.
At its top, the basilar artery is divided into two terminal branches: the posterior cerebral arteries.
So behind: the vertebrobasilar system, and ahead: the two internal carotid arteries.
5. View of the underside of the brain
The two internal carotid arteries exit from the skull outside the anterior clinoid process and outside the optic nerve or II cranial nerve. They give immediately above the roof of the cavernous sinus a first artery, under the optic nerve which reaches the optic canal: the ophthalmic artery.
It is the first supra-cavernous collateral of the internal carotid.
The second collateral is the posterior communicating artery which arises from the posterior surface of the internal carotid artery and which joins towards the rear, after having crossed the ipsilateral optical strip, the posterior cerebral artery.
This posterior communicating artery has a very important functional role because it anastomoses the carotid system to the basilar system.
Anteriorly the internal carotid gives a first terminal branch: the anterior cerebral artery which passes above the optic nerve to join the interhemispheric fissure.
These two anterior cerebral arteries are anastomosed by the anterior communicating artery.
So on the underside of the brain we have an anastomotic system connecting the carotid system and the basilar system. This is the Willis polygon. The system permanently makes it possible to compensate for carotid or vertebral failures for a homogeneous vascularization.
This polygon is the site of multiple variations from one individual to another.
The internal carotid artery gives rise to a second terminal branch: the middle cerebral artery. It is born laterally to walk in the lateral fissure. Its course is initially transverse, oblique outwards and slightly forward; then it is vertical, oblique up and back, moving over the insula.
6.1. The cerebral arteries
They give up a certain number of superficial collateral.
• The anterior cerebral artery:
• first branch supporting the anterior and internal part of the frontal lobe;
• second branch following the calloso-marginal groove in its anterior two thirds;
• third terminal branch running above the corpus callosum: the pericallosal artery;
• plus a small branch originating from the anterior cerebral artery: the striated Heubner artery for the vascularization of the anterior, inferior and internal part of the frontal lobe;
• the middle cerebral artery: it provides collateral in the depth of the sylvian valley, leaving the lateral fissure to become superficial, vascularization of the anterior two thirds and external two thirds of the cerebral hemispheres is ensured;
• the posterior cerebral artery: it ensures the vascularization of the temporal lobe in its internal surface, of the occipital in the internal and lateral surfaces.
6.2. The vertebrobasilar system gives rise to superficial branches.
• From the vertebral artery arises a posterior spinal artery, an anterior, branches for the meninge; and especially a postero-inferior cerebellar artery ensuring the vascularization of the underside of the cerebellum.
• Above the basilar artery arises the middle cerebellar artery bypassing the bridge to irrigate the cerebellum;
• Above, the basilar artery gives rise to the postero-superior cerebellar artery, it arises under the posterior cerebral artery, for the upper part of the bridge and the cerebellum. Between the posterior cerebral and the postero-superior cerebellar we see the appearance of the third cranial nerve going towards the roof of the cavernous sinus.
6.3. Deep branches:
6.3.1. Anterior / striated central arteries:
• The anterior striated arteries: they arise from the anterior cerebral, penetrate the anterior perforated space to ensure the vascularization of the anterior part of the striated nuclei (caudate and lenticular nuclei).
• The internal striated arteries arise from the middle cerebral, at its origin, to reach the middle part of the lenticular nucleus.
• The external striated arteries arise from the proximal portion of the middle cerebral and enter the lateral part of the lenticular nucleus. Part of this contingent can cross it completely to gain, forward and up, the head of the caudate nucleus.
6.3.2. Middle central arteries
They arise from the posterior communicator and move inwards for: the floor of the V3, chiasma, optical strip proximally, pituitary rod, and mammillary tubercles.
6.3.3. Posterior central arteries
They arise from the posterior cerebral, penetrate the posterior perforated space to vascularize the posterior two-thirds of the thalamus and the geniculate bodies.
Pallidum and internal capsule are absolutely not taken care of by the striata.
6.3.4. Anterior choroidal arteries
They arise directly from the internal carotid artery, above the posterior communicating artery. They go back and out, towards the anterior end of the lateral face of the slit of Bichat. They accompany the lateral choroid plexuses to the hole of Monro and this choroidal artery anterior participates in the vascularization of the posterior arm of the internal capsule and at the tip of the lenticular nucleus.
6.3.5. Posterior choroidal arteries
They arise from the posterior cerebral, outside the opening of the posterior communicating artery. It is lateral and supplies the lateral choroid plexus up to Monro’s hole.
6.3.6. Posteromedial choroidal arteries:
They arise from the postero-superior cerebellar artery, pass near the pineal gland, and supply the median choroid plexus.