510 'Journal of Comparative Neurology and Psychology. 



Fig. I. Diagram showing the normal direction of the blood current in the basal arteries, and how 

 completely separated are the carotid and the vertebral system by the balance in the middle of the posterior 

 communicating arteries. At the same time in normal conditions the two hemispheres are almost as 

 distinctly supplied, owing to the balance in the anterior communicating artery, and the direction of the 

 current in the posterior cerebral arteries. The cerebellar circulation is not so distinctly separated on 

 the two sides and the anastomoses between the cerebellar arteries are more complete than those of the 

 cerebrum. 



Fig. 2. Diagram showing the most common variations in the circle of Willis, etc. (A) Enlargement 

 of one anterior cerebral artery with corresponding small size of the opposite artery; (B) double anterior 

 communicating artery; (C) common trunk made by junction of the two anterior cerebral arteries, with 

 division at the genu callosi; (D) enlargement of one, or both posterior communicating arteries which 

 then go to supply the region of the posterior cerebral arteries which are abnormally small; (£) doubling 

 of the superior cerebellar artery; (F) partial doubling of the basilar artery; (G) common trunk giving 

 off the anterior, and posterior inferior cerebellar arteries from the basilar; {H) common trunk giving off 

 these two arteries from a vertebral artery; (/) small size of one vertebral artery with corresponding large 

 size of the opposite artery; {K) large size of a vertebral artery before giving oflf the posterior inferior 

 cerebellar artery. 



Fig. 3. Anomalous arrangements of arteries shown by Cases 2000, 2004, 2057 and 2063. 



