Comparative study of formation of circle of Willis in human and sheep brain

In sheep a well-developed four-sided arterial rete called 'Rete Mirabile cerebri' (RMC) was noted at the base of the brain in inter-peduncular fossa. Internal carotid arteries were either vestigial or absent in sheep. Two emergent arteries arising from steady pool of blood from RMC entered the inter-peduncular fossa. These arteries ascended on either side of infundibulum of pituitary gland and then diverged in cranial and caudal directions to form the Circle of Willis.