188 I. W. BLACKBURN 



"Note on the arterial supply of the Brain in Anthropoid Apes," 

 these observers state: 



Among features of human character pertaining to the anthropoid 

 apes, we find one not hitherto recorded, namely the existence in the cere- 

 bral arterial supply of a Cir cuius Willisii resembling that of man. In 

 these highest apes, as in man, an anterior communicating artery quite 

 frequently completes the circulus in front; 



and, quoting Parsons, they add : 



In all of the lower mammals which I have examined, including the 

 Platyrrhine monkeys, Cercopithecus, etc., I find no anterior communi- 

 cating artery. 



In all of the lower forms of mammals instead of the 

 pair of anterior cerebral arteries, there is but a single azygos 

 vessel, which in its course forward gives off branches to the right 

 and left hemispheres respectively. These observers examined 

 six brains of chimpanzees, and one orang-outang. Of the six 

 chimpanzees, in five the circulus had the human type. In one 

 of these two anterior cerebral trunks were united or fused into 

 a common trunk about four mm. in length. In the orang-outang 

 also the circulus was completed by an anterior communicating 

 artery, and in one of the six chimpanzees there was a single azygos 

 anterior cerebral artery as in the lower mammalian forms. This 

 work, which is in accord with the writer's limited experience, 

 shows that the existence of an azygos vessel in place of the anterior 

 communicating artery and the two anterior cerebrals is the rule 

 in the lower mammals, and that the form found in the higher 

 anthropoid apes and in man is a phylogenous development while 

 it also suggests that in view of the frequency of anomalous forms 

 in the anterior cerebrals, that this phylogeny is as yet unstable. 



In these arteries, anomalies of development are by all odds 

 more frequent than in any other sets of vessels. Windle in 200 

 cases found 8 of fusion, 9 median anterior cerebral arteries, and a 

 large number of anomalies of the anterior communicating artery; 

 in my own 220 cases, fusion of the vessels was found in 7 cases, in 



