Blackburn, Anomalies of Encephalic Arteries. 499 



ment of the posterior communicating arteries is correlated with 

 small size of the posterior cerebral trunks at their first part or 

 origin from the basilar. When but one of these arteries is en- 

 larged it is most commonly the right. Quain found the right 

 artery enlarged in 5.5 per cent of the cases examined; the left in 

 4.5 per cent; both in 2 per cent. 



In all these cases of enlargement of the posterior communicating 

 arteries the main blood supply to the posterior cerebral territory 

 comes from the internal carotid artery, but I think it a mistake 

 to regard even high degrees of this anomaly as instances of origin 

 of the posterior cerebrals from the carotid system. Often the 

 posterior cerebral is represented by a mere thread, sometimes this 

 may be impervious or even absent, but in none of my cases have 

 I been convinced of the carotid origin of the posterior cerebral. 



Hyrtle reports a case in which the middle cerebral was given off 

 by the posterior cerebral a condition not so readily explained, but 

 it must be extremely rare. The posterior communicating artery 

 is not infrequently wanting, fails to join the posterior cerebral, or 

 ends in a few filiform branches about the crus. Quain gives it 

 as absent on the right side in 4.5 per cent; on the left in 6.5 per 

 cent; and on both sides in 1.5 per cent. 



The origin of the posterior communicating arteries from the 

 middle cerebral rather than from the internal carotid I deem 

 unimportant, as the former vessel is merely a continuation of the 

 latter without definite dividing line, though the posterior communi- 

 cating arteries arise inside of the origin of the anterior choroids, 

 the latter arising from the terminal part of the internal carotid. 



The posterior communicating artery of the left side was abnor- 

 mally small in seven of my cases; that of the right side in three; 

 both unusually small in four cases. The vessel was totally absent 

 on the right side in one case. The right vessel was enlarged 

 twenty-six times; the left sixteen times; and both were enlarged in 

 twenty-three cases. In all of these cases the corresponding pos- 

 terior cerebral arteries were below the normal in size. In several 

 instances there were slight and inadequate anastomoses with the 

 basilar through small and abnormal branches representing the 

 posterior cerebral arteries. Nos. 2099, 2159 and 2177. 



WiNDLE found these arteries normal in 175 of his cases. In 

 twenty-eight cases the right was the larger; in fifteen it was the 

 left; both were abnormally small in seven cases. He gives these 



