Changes

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.  

===Blood Supply to the Circle of Willis===

===Blood Supply to the Circle of Willis: The Basics===

In order to provide details of the species differences regarding blood supply to the brain, it is necessary to highlight the main areas of anatomy that are important within and around the circle of Willis. The following informatino is very generalised and is used to provide knowledge and background to the species specifics given below.

In order to provide details of the species differences regarding blood supply to the brain, it is necessary to highlight the main areas of anatomy that are important within and around the circle of Willis. The following informatino is very generalised and is used to provide knowledge and background to the species specifics given below.

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The main blood supply to the circle is via the paired internal carotid arteries and the basilar artery. The basilar artery receives blood from the ventral spinal artery and the vertebral artery (the vertebral artery is a branch of the subclavian artery running through the vertebral foramina of C1 - C6). The internal carotid artery receives blood supply from the external carotid artery, the common carotid artery and in some species also the vertebral artery. In some species  

The main blood supply to the circle is via the paired internal carotid arteries and the basilar artery. The basilar artery receives blood from the ventral spinal artery and the vertebral artery (the vertebral artery is a branch of the subclavian artery running through the vertebral foramina of C1 - C6). The internal carotid artery receives blood supply from the external carotid artery, the common carotid artery and in some species also from the vertebral artery via the occipital artery. The external carotid artery itself can also receive blood from the maxillary artery. In some species the maxillary artery is also directly able to supply the internal carotid artery, bypassing the external carotid artery via an anastomising ramus linking the internal carotid and maxillary arteries. This maxillary anastomising ramus allows blood to flow through the maxillary rete mirabile which is a network of vessels located within the cavernous sinus, facilitating the cooling of blood temperature and reducing fluctuations from pulsatile blood flows (see rete mirabile section below for further details on function). As mentioned above, the vertebral artery can also supply the internal carotid artery via the occipital artery but this too can be bypassed so that the vertebral artery can directly supply the internal carotid artery via a ramus to the internal carotid directly from the vertebral artery.

<br> <u>Note</u>: The basilar artery and middle cerebral arteries, though they supply the brain, are not considered part of the circle.  

<br> <u>Note</u>: The basilar artery and middle cerebral arteries, though they supply the brain, are not considered part of the circle.