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 ease in my specimens (see fig. 3). 



y External carotid 



Ventral aorta -- 



Right subclavian^ 

 artery 



Right vertebral 



artery 



Right--''' 

 pulmonary 

 artery 



Tmvnk 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 Jeft 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 



