456 
Philippine Journal of Science 
1919 
arteries. The trunks of the first six pairs are lost so that the vertebrals 
take their origin with the subclavians from the seventh pairs of inter- 
segmental arteries. 
Having gained an idea of the normal origin of the vertebral 
arteries, anatomically and embryologically, three different pos- 
sibilities would probably account for the abnormal origin of the 
left vertebral artery as hereinbefore described. 
One of the possibilities is the persistence of the second aortic 
arch in forming the proximal portion of the internal carotid. 
The ventral aortic stem between the second and third arches 
takes the place of the left common carotid. The third aortic 
arch, which eventually becomes the left vertebral artery, shifts 
downward and laterally comes into direct continuity with the 
fourth arch, which becomes the future permanent arch of the 
aorta. Under this possibility it will be seen that the proximal 
part of the left vertebral artery will be found situated between 
the left common carotid and the left subclavian artery, as was 
actually the case in my specimens (see fig. 3). 
, External carotid. 
Ventral aorta 
Right subclavian 
artery 
Right vertebral 
artery - 
Right'-"'' 
pulmonary 
artery 
Trunk of pulmonary 
artery 
— Internal carotid 
Common carotid 
Aortic arch 
— Ductus arteriosus 
-- Vertebral artery 
"Subclavian artery 
Left pulmonary 
artery 
"’Dorsal aorta 
Fig. 3. Diagram, showing one possible abnormal origin of left vertebral artery. 
Another not remote possibility would be the persistence of the 
proximal part of the left dorsal aortic stem between the third 
and fourth aortic arches and the main trunk of the sixth or one 
of the more cephalic dorsal intersegmental arteries, the two 
