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== Circle of Willis  ==

== Circle of Willis  ==

[[Image:376px-Circle of Willis en.svg.png|thumb|right|150px|Generalised Structure of the Circle of Willis, Rhcastilhos, 2007, WikiMedia Commons]]

[[Image:376px-Circle of Willis en.svg.png|thumb|right|150px|Generalised Structure of the Circle of Willis, Rhcastilhos, 2007, WikiMedia Commons]]

Blood is supplied to the brain from a ventral arterial supply in all species, from a circle of arteries called the Circle of Willis (also called the ''cerebral arterial circle'' or ''arterial circle of Willis'') which lies ventrally to the hypothalamus where it forms a loose ring around the infundibular stalk. Although the appearance of the circle of Willis is fairly constant amongst mammals, the sources of blood supply to the circle and the direction of flow around the circle are very species specific. Blood is supplied to the brain by the ''internal carotid artery'' in dogs and horses whilst in other domestic species the main blood supply is from branches of the ''maxillary artery''.  The Circle of Willis is made up of the following arteries;  

Blood is supplied to the brain from a ventral arterial supply in all species, from a circle of arteries called the Circle of Willis (also called the ''cerebral arterial circle'' or ''arterial circle of Willis'') which lies ventrally to the hypothalamus where it forms a loose ring around the infundibular stalk. Although the appearance of the circle of Willis is fairly constant amongst mammals, the sources of blood supply to the circle and the direction of flow around the circle are very species specific. Blood is supplied to the brain by the ''internal carotid artery'' in dogs and horses whilst in other domestic species the main blood supply is from branches of the ''maxillary artery''.  The Circle of Willis is made up of five main pairs of vessels;  

*Left and right inferior cerebral arteries

*Rostral Cerebral Arteries: supply the medial aspect of the cerebral hemispheres.

*Anterior communicating artery

*Middle Cerebral Arteries: supply the lateral and ventrolateral aspects of the cerebral hemispheres.

*Left and right internal carotid arteries

*Caudal Cerebral Arteries: supply the occipital lobes.

*Left and right posterior cerebral arteries

*Rostral Cerebellar Arteries: supply the rostral aspects of the cerebellum

*Left and right posterior communicating arteries

*Caudal Cerebellar Arteries: supply the caudal and lateral aspects of the cerebellum.

The arrangement of the Circle of Willis means that if one part of the circle becomes blocked or narrowed (stenosed), or one of the arteries supplying the circle is stenosed, blood flow from the other blood vessels can continue to provide a continuous supply of blood to the brain.  

The arrangement of the Circle of Willis means that if one part of the circle becomes blocked or narrowed (stenosed), or one of the arteries supplying the circle is stenosed, blood flow from the other blood vessels can continue to provide a continuous supply of blood to the brain.