xv, 5 Canizares: Abnormalities of Vertebral Artery 457 



together representing the left vertebral artery which, in the 

 final processes of shifting, would come to arise directly from the 

 arch of the aorta between the left common carotid and the left 

 subclavian arteries (see figs. 3 and 4). 



A third and last possibility is that the left vertebral artery, 

 instead of arising from the dorsal rami of the seventh interseg- 

 mental artery together with the subclavian, originated independ- 

 ently from the sixth intersegmental artery, without the per- 

 sistence of the proximal part of the dorsal aortic stem between 

 the third and fourth aortic arches if the sixth intersegmental 

 artery was located caudad to them (see figs. 4 and 5). 



Ventral aorta 



Right subclavian 

 artery "— 



Right pulmonary 

 artery 



/External carotid 



-Internal carotid 



Common carotid 

 , Aortic arch 



Vertebral artery 



r~ Ductus arteriosus 



Subclavian artery 



Left pulmonary 

 artery 

 ' Trunk of pulmonary 

 artery 



y—* Dorsal aorta 



Fig. 4. Diagram, showing' another possible abnormal origin of the left vertebral artery. 



With regard to the abnormalities in the point of entrance it 

 was found that the foramen transversarium in all the cervical 

 vertebrae below the level of entrance of the vessels was patent. 



Keith (4) says that the cervical transverse foramina are pro- 

 duced by perforations of the transverse processes, while still 

 fibrous, by the corresponding intersegmental arteries, the anas- 

 tomosis of which eventually formed the vertebral artery. 



Therefore, in cases where the point of entrance was at an 

 abnormally higher level, it would seem probable that the verte- 

 bral artery was formed by fusion of the persisting and corre- 



