190 I. W. BLACKBURN 



10 the right artery was very small, and the opposite carotid artery 

 sent the blood supply to the two hemispheres through an enlarged 

 anterior communicating artery, and in 6 cases the left artery was 

 the smaller, and the main blood supply came from the right caro- 

 tid system; in a large percentage of cases, anomalies of the com- 

 municans were found. It is possible that in many of the apparent 

 anomalies of the anterior cerebral arteries with disproportion 

 in size of the trunks beyond the anterior communicating artery, 

 the larger vessel may be a partial reversion to the primitive type ; 

 that is, a median anterior cerebral unusual only in place of origin. 

 In fact, it is not at all uncommon for the larger of the two trunks 

 to send branches to the mesial surfaces of both hemispheres. 



It may then be granted that the lower types are being developed 

 in the phylogeny of the races, and such being the case, reversion 

 to the primitive type may now and then be expected. It has 

 seemed to the writer that the development and reversions might 

 be shown graphically in the accompanying drawings, nearly all 

 of which are drawn from actual specimens. In Fig. 1, a repre- 

 sents the primitive form as met with in the lower mammals; 

 b shows the fusion of the anterior cerebrals with the remnant 

 of the azygos vessel at the end of the junction; c a later stage 

 of the development, with the remnant of the azygos vessel; d 

 shows the shortening of the place of junction, and e shows a fully 

 developed anterior communicating artery. Fig. 2 shows the occa- 

 sional reversions met with ; a gives the type of arteria termatica 

 of Wilder; b gives the type of termatic artery which arises from 

 the end of the short fusion of the two vessels; c shows the most 

 common form of median anterior cerebral artery; d, that in which 

 the anomalous artery is the largest of the three, and e, the almost 

 complete reversion to the lower mammalian type; e, however, has 

 not actually been seen by the writer, while d is common. 6 



It is suggested by these forms that in the higher phylogenetic 

 development of the brain, the greater size of the frontal lobes has 



Quain says: "The two arteries have also been seen united in a single trunk, 

 which runs in the longitudinal fissure, giving off branches to both hemispheres." 

 The writer has recently found one of these vessels, hence the series may be regarded 

 as complete. 



