SOCIETY OF THE UNIVERSITY OF ABERDEEN. 



75 



common to the two carotids and the left snbclavian is a rather more 

 difficult problem. It is probably due in the first instance to absorp- 

 tion of the fourth left aortic arch and the consequent shifting of the 

 ductus arteriosus. If the left subclavian artery has grown out, as it 

 ought to do, from the dorsal end of the fourth left aortic arch, then 

 the ductus arteriosus must represent the dorsal end of the fifth left 

 aortic arch plus the dorsal root of the fourth arch, which would, of 

 course, be altogether abnormal. If, on the other hand, the left sub- 



Fig. 7. Diagram to illustrate the evolution of the great vessels in the case of the right twin. 



clavian artery has grown out from the dorsal end of the fifth arch, 

 the fusion having involved not only the fourth left arch but also its 

 dorsal root, then the ductus arteriosus is exactly where it should be, 

 but the subclavian, as regards its third part, is somewhat abnormally 

 developed. Whichever explanation be adopted it involves a slight 

 deviation from the type. The chief variations in the great thoracic 

 vessels of the right twin are, therefore, chiefly referable to the fourth 

 left aortic arch, that is to say to the arch which has been concerned 

 with the fused left upper limb of that twin. 



