55 



the extremely rare variation of failure of the aa. vertebrales to unite 

 at the caudal border of the pons to form the a. basilaris. How exceed- 

 ingly uncommon this variation is it will be one of the objects of this 

 communication to show. 



The abnormality which we propose to describe occurred in a male 

 aged 64. Born in Switzerland he emigrated to Australia many years 

 ago and died in the Melbourne Hospital and was thence admitted to 

 the Anatomical Department of this university. On opening up the 

 skull and removing the brain our attention was directed by the students 

 dissecting the part to the fact that the two aa. vertebrales altogether 

 failed to unite to form the a. basilaris. As we recognised the general 

 rarity of the condition at a glance a careful examination was made 

 of the two aa. vertebrales and the a. basilaris pending a systematic 

 examination of the literature. 



The right a. vertebralis arose from the right a. subclavia 1 cm 

 from the bifurcation of the a. anonyma and had , at its origin a 

 diameter of 3.5 mm. It entered the foramen transversarium of the 

 sixth cervical vertebra and pursued thence a perfectly normal cranial 

 course. At the foramen transversarium the diameter had diminished 

 to 3 mm, and thereafter progressively diminished to 2.8 mm opposite 

 the artic. atlantoepistrophica, and to 2.5 mm within the cranium. On 

 entering the cranial cavity the vessel did not unite with its fellow of 

 the opposite side but passed laterally on the caudal surface of the 

 right hemisphaerium cerebelli to about 2.5 cm from the median plane, 

 where it turned dorsally and finally medially as the a. cerebelli inferior 

 posterior. The vessel thus terminated as an artery of the cerebellum 

 without establishing any communication whatsoever with the a. verte- 

 bralis of the left side. (See Fig. 1.) 



The left a. vertebralis arose from the left a. subclavia at the 

 greatest convexity of its cervical curve and at about the same distance 

 from the median plane as on the right side. Its intervertebral course 

 was perfectly normal and was exactly similar to the corresponding 

 vessel of the right side. In view of what will be said later it is 

 important to compare the diameters of the vessels of the two sides: 



Right Left 



Diameter at origin 3.5 mm 6.0 mm 



Diameter at foramen transversarium of C. V. 6 3 ., 4.2 „ 

 Diametej opposite artic. atlantoepistrophica 2.8 „ 7.5 ,, 



Diameter within cranium 2.5 „8 „ 



On entering the cranium the left a. vertebralis inclined towards 

 the median plane which it gained at the caudal border of the pons. 



