Comparative anatomy of the circle of Willis in man , cow , sheep , goat , and pig

The circle of Willis (cW), a potential collateral pathway at the base of brain, is a subsidiary network of vascular channels that stabilises cerebral blood flow when principal conduits fail. Considerable variability exists in the anatomy of cW with frequent asymmetry while an ideal configuration is seen in a minority of cases. The importance of this circular anastomosis becomes evident while determining the adequacy of brain circulation in operations of cerebral aneurysms and ligation of carotid artery. The adequacy of recovery or lack of recovery after vascular occlusion partly depends upon the anatomical state of cW – normal or anomalous. 7% incidence of hemiplegia following carotid artery occlusion has been attributed to congenital absence of posterior communicating artery [1]. Hence a good knowledge of the anatomy of cW and its variations becomes essential. The incidence of anomalies is varied, ranging from 1173% [2] or even more. It gains importance in carotid ligation, reperfusion in case of obstruction of feeding vessels, surgical treatment of cerebral aneurysms whose post-operative recovery depends on the co-existence of anomalies in the circle which determines the adequacy of collateral circulation. For implementing new surgical techniques and invasive procedures, their operative and post-operative success have to be asserted before hand by doing experiments and trials. For this purpose, animal models have to be created which simulate the humans. Various animals have been used in research to study the effects of drugs on cerebral circulation, quantitative analysis of cW and its branches and to try out new surgical techniques. Animal models have been created to elucidate the post treatment pathologic and hemodynamic changes. Animals with experimentally induced atherosclerosis have also been used for research purposes [3]. Anatomical knowledge about similarities and differences in the arterial pattern of humans and other mammals used becomes essential for comparison of the results and their implications in man. The information available regarding the pattern of circulus arteriosus cerebri in mammals in the recent past is inadequate. Hence, there is a need to study the anatomy of cerebral circulation including cW in animals, to determine the similarities and differences. Research work regarding this being scant, instigated the pursuance of the present study. A comparative study was done and the following parameters (objectives) were studied: 1) Vessels forming the cW, 2) Diameters of the vessels forming the cW, 3) Variations noted in the pattern of the cW.