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

The circle of Willis in animals can be used to create models that simulate man in order to conduct experiments and trials. For this, a detailed comparative morphological study becomes essential. The objectives of the present study are to note the configuration of circle of Willis, measure the diameter of vessels forming it, and observe for variations. The study included ten brain specimens each of cow, sheep, goat, pig and man. The internal carotid artery formed an integral part but the posterior cerebral artery was not so in the animal specimens studied when compared to that of man. The anterior communicating artery was in the form of network in the animals while it was a single vessel in man. The posterior communicating artery was proportionately larger in animals than in man. The basilar artery was seen to originate from the circle of Willis in the ruminants. Absence of anterior communicating artery was the most common anomaly noted among the animals studied (10-20%). In man, the anomalies were varied in both anterior (50%) and posterior (80%) halves of the circle. The reason for the above differences noted and for anomalies occurring more in humans can be attributed to presence or absence of contribution of the vertebro-basilar arterial system to the circle of Willis and its consequences.