THE BRAIN 



series of anastomoses which take place at the base of the 

 brain, as well as by the anastomoses of their terminal branches 

 in the pia mater on the surfaces of the hemispheres. 



The more striking series of anastomoses are those at the 

 base of the brain ; they constitute the circulus arteriosus 

 (Willis), and the arteries which take part in the formation of 

 the circle lie in the cisterna interpeduncularis and the cisterna 

 chiasmatis. 



Circulus Arteriosus (O.T. Circle of Willis). The series 

 of anastomoses which forms the arterial circle lies at the 



.Anterior cerebral artery 



Anterior communicating 



Internal, carotid 

 W 

 Middle cerebral 



Antero-lateral central arteries 

 Posterior communicating 



Postero-median central arteries 

 Posterior cerebral artery 



Superior cerebellar artery 



FIG. 144. Diagram of the Circulus Arteriosus. 



base of the brain, in the deep hollow anterior to the pons 

 and around the optic chiasma. The so-called circle has, 

 in reality, a heptagonal or hexagonal outline, and the vessels 

 which compose it lie, as already stated, in the cisterna inter- 

 peduncularis and the cisterna chiasmatis. Anteriorly it is 

 formed by the anterior communicating artery, which links 

 together the two anterior cerebral arteries. On each side is 

 the posterior communicating artery, connecting the internal 

 carotid (from which the anterior cerebral springs) with the 

 posterior cerebral. The arterial ring is completed posteriorly 



