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 
' Ductus arteriosus 
Subclavian artery 
Left pulmonary 
artery 
' Trunk of pulmonary 
artery 
"-'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- 
