The arterial blood supply to the forebrain, midbrain and to the rostral cerebellum comes from the circulus arteriosus cerebri also known as circle of Willis. This surrounds the hypophysis and the optic chiasm on the ventral surface of the encephalon. In the dog, three arteries supply the circulus: the basilar artery and the two internal carotid arteries. The hindbrain receives blood from branches of the basilar artery.
The basilar artery is a rostral continuation of the ventral spinal artery. In its course on the ventral aspect of the medulla oblongata, branches off the caudal cerebellar artery, the labyrinthine artery and pontine arteries. In the cat, this artery does not supply the circulus as the flux of blood is rostrocaudal.
The internal carotid artery enters the caudal carotid foramen through the tympano-occipital fissure to run in the carotid canal to leave through the external carotid foramen, makes a loop and reenters the cranial cavity through the internal carotid foramen. It receives the ascending pharingeal arteri before reentering the cranial cavity. In the cat, the internal carotid artery is occluded. However, intracranially, there is an internal carotid artery that receives blood through the ascending pharyngeal artery (via the carotid foramen), and from the rete mirabile (of the maxillary artery) via the orbital fissure.
In cats, the internal carotid artery is occluded. However, intracranially, there is an internal carotid artery that receives blood through the ascending pharyngeal artery (via the carotid foramen), and from the rete mirabile (of the maxillary artery) via the orbital fissure.
The rostral cerebral artery is a terminal branch from the internal carotid artery. It forms the rostral part of the circle of Willis. It courses dorsally to the optic chiasm, surrounds the genu of the corpus callosum and continues along its dorsal surface to anastomose with the caudal cerebral artery. Arterial bridges may be present between the left and right rostral cerebral arteries. An important one is the rostral communicating artery, located rostrally to the optic chiasm. Other bridges may be present at the level of the genu of the corpus callosum. The rostral cerebral artery supplies the medial cortical surface of the frontal cortex, the cingulated gyrus and the gyri bordering the longitudinal fissure on the rostral two thirds of the surface of the cerebrum. The internal ethmoidal artery and the internal ophthalmic artery are branches of the rostral cerebral artery. The internal ethmoidal artery reaches the cribriform plate of the ethmoidal bone where it anastomoses with branches of the external ethmoidal artery. The internal ophthalmic artery follows the optic nerve through the optic canal to the orbit where it anastomoses with branches of the external ophthalmic artery.
The middle cerebral artery is the largest artery in the brain. It is a branch of the arterial circle of the brain near the internal carotid artery (some authors point out that the middle cerebral artery is a terminal branch of the internal carotid artery). It courses in front of the piriform lobe and is directed to branch off the lateral surface of the cerebral hemisphere. Between the olfactory tubercle and the piriform lobe, the middle cerebral artery divides into two branches. The rostral branch branches off the striated arteries in a vertical direction to vascularize the basal nuclei and the internal capsule. Patological disturbances of the striated arteries may not result in hemiplegia but may cause compulsive circling towards the side of the lesion.
The caudal cerebral artery is a branch of the caudal communicating artery. It is located rostral to the oculomotor nerve. It extends over the ventral surface of the crus cerebri to continue, dorsolaterally, through the cerebral peduncle. Right after originating from the caudal communicating artery, it divides into a rostral and a caudal branch. The rostral branch extends between the optic tract and the medial geniculate body to reach the roof of the third ventricle and the splenium of the corpus callosum, where it sends branches to the occipital cortex and anastomoses with the rostral cerebral artery. In its ascending course, it surrounds the lateral geniculate body and gives rise to the caudal choroidal artery. The rostral branch of the caudal cerebral artery supplies blood to the pineal gland, the choroid plexus of the third ventricle, geniculate bodies, the brachium of the rostral colliculus, the medial surfaces of the thalamus, and to the medial surface of the caudal part of the brain and the gyri bordering the longitudinal fissure in the caudal third of the dorsal surface of the brain. The caudal branch of the caudal cerebral artery vascularizes the diencephalon and the rostral midbrain.
The rostral cerebellar artery is a branch from the caudal communicating artery. It courses caudally to the oculomotor nerve, and dorsocaudally along the lateral surface of the pons and the middle cerebellar peduncle to supply the rostral half of the cerebellum and the caudal mesencephalon.
The caudal cerebellar artery is a branch of the basilar artery at the level of the medulla oblongata. It is located rostrally to the rootlets of the hypoglossal nerve. It courses dorsally, along the lateral surface of the medulla oblongata and the caudal cerebellar peduncle, to vascularize the caudal half of the cerebellum. After passing caudally the caudal cerebellar peduncle, gives rise to the choroid plexus of the fourth ventricle.
The labyrinthine artery originates from the basilar artery and ascends caudal to the trapezoid body, and accompanies the vestibulocochlear nerve through the internal acoustic meatus to vascularize the inner ear.
The dura mater is vascularized by the rostral, middle and caudal meningeal arteries. The rostral meningeal artery is a branch of the external ethmoidal artery close to the cribiform plate inside the cranial cavity. The middle meningeal artery is the most extensive meningeal artery. After branching from the maxillary artery, enters the cranial cavity through the oval foramen, runs caudosorsally and bifurcates into rostral and caudal branches. It supplies the dura mater that covers the temporal and parietal cortices. The caudal meningeal artery is a branch of the occipital artery that enters the cranial cavity through the mastoid foramen. It supplies the dura mater of the dorsocaudal cranial cavity and the tentorium cerebelly.
Vascularization of the choroid plexuses
The arterial supply to the choroid plexuses of the lateral ventricles comes from the rostral choroidal artery. This is a branch that detaches from the middle cerebral artery or the caudal communicating artery near the internal carotid artery. It enters the lateral ventricle and nourishes the choroid plexus.
The caudal choroidal artery is a branch of the caudal cerebral artery that supplies blood to the choroid plexus of the third ventricle.
The choroid plexus of the ventricular curate receives arterial blood from the caudal cerebellar artery once it has passed over the caudal cerebellar peduncle.
Vascularization of the hypophysis
The hypophysis is surrounded by an arterious circle formed by the caudal intercarotid artery and the rostral intercarotid artery. The first is an extense artery that is located on the laterocaudal aspect of the adenohypophysis. The second is located on the rostral aspect of the adenohypiphysis, and runs between the internal carotid arteries prior to join the circulus arteriosus cerebri. Both arteries are located in the subarachnoid space. The tuber cinereum, the infundibulum, and the neural lobe receive arterial blood supply from the rostral intercarotid artery. Capillaries from these sources also serve to allow releasing factors from the hypothalamus to the adenohypophysis.
The adenohypophysis is vascularized by capillaries from the caudal intercarotid artery forming the caudal hypophyseal arteries.
The venous drainage empty into the cavernous and intercavernous sinuses.
Vascularization of the thalamus
The rostral and dorsomedial thalamic nuclei are vascularized by capillaries coming from the caudal communicating artery. Small vessels from the posterior cerebral artery supply the rest of the thalamus, and extend also to the rostral and dorsomedial nuclei.
 Described by Johan Vesling in 1647 and named by Thomas Willis in 1664.
 Page 1769 of "The anatomy of the domestic animals" by S. Sisson and J. D. Grossman 5th ed.).