56 



Between this point and the cranial border of the pons the left a. verte- 

 bralis, or more correctly perhaps the a, basilaris described a gentle 

 curve on the right side of the median plane with its maximum con- 

 vexity 1 cm to the right of the median line; passing thence to the 

 left the vessel regaimed the middle line at the cranial border of the 

 pons and there divided in a perfectly normal manner into two aa. 

 cerebri posteriores. (See Fig. 1.) It is thus clear that the a. basi- 

 laris was not formed by the junction of the two aa. vertebrales but 

 was a direct continuation of the left a. vertebralis, and that the 

 circulus arteriosus (Willisi) was completed posteriorly and was in all 

 respects perfectly normal. 



N. opticus 



Chiasma opticum 



Infundibulum 



Corpus mamillare 



Pons, caudal border 



Medulla oonlbgata 



Hemisphaerium cerebelli 



A. cerebri anterior 



A. communicans anterior 



A. carotis interna sinistra 

 A. communicans posterior 

 A . cerebri posterior 



A. basilaris 



A. vertebralis dextra 

 A. vertebralis sinistra 



Fig. 1. Dioptrographic drawing of the abnormal A. basilaris. Natural size. 



We have already stated that we recognised in a general way that 

 failure of the union of the two aa. vertebrales to form the a, basi- 

 laris was rare, but we were not prepared, until we had made a com- 

 plete survey of the literature of the subject, to find that the condition 

 is apparently one of the very rarest things in anatomical science and 

 that there has been recorded only one case at all similar, that of 

 Batujeff (5). 



Of the standard anatomical textbooks Morris (6) and Quain (7) 

 make no mention whatsoever of the condition. Piersol (8) states 

 that "very rarely the two vertebrals fail to unite to form a single 

 median basilar, that artery being thus represented by two longitudinal 

 trunks united by transverse anastomoses". 



