Changes

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On the left side of the brain in these species there is a distinct missing connection. Where the vertebral artery supplies the occipital artery and this then supplies the internal carotid, this connection is missing so the occipital is only able to join the external carotid and the common carotid arteries. Similarly the vertebral artery ramus is also missing so there is no direct blood supply to the internal carotid from the vertebral artery and also no associated rete mirabile. The maxillary ramus supplying the internal carotid is still patent on the left side. Maxillary blood is distributed to all of the brain except the caudal part of medulla oblongata, which is supplied by vertibral blood. Therefore blood supply to the circle of Willis in ovines and felines is not bi-laterally symmetrical.

 

Only channel 3 supplies the arterial circle and unlike in the dog, this channel has a rete mirable. Channel 2 carries blood away  from the arterial circle. Maxillary blood is distributed to all of the brain except the caudal part of medulla oblongata, which is supplied by vertibral blood.

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