CONGENITAL MALPORMATIONS. 297 



(fig. 95). That portion of the arch which intervenes between 

 the left subclavian and the ductus arteriosus Botalli, is termed the 

 isthmus aortcB. 



Departures from this order during the early development of 

 the great vessels are not infrequent. Thus the se^^tum between 

 the innominate and loft carotid may go beyond the arch, may 

 extend into the aortic bulb. We then get a double ascending 

 aorta, whose smaller right limb is nothing but the innominate 

 trunk which has become independent. 



Should the separation of the left carotid from the left sub- 

 clavian remain incomplete, the two vessels spring from a common 

 trunk, a left innominate artery. 



Again, the tliird right arch (counting from below) may be 

 converted, not into the right subclavian, but into the right 

 carotid, the fourth right arch into the left carotid, and the 

 fourth left arch into the left subclavian, so that we have a super- 

 numerary trunk on the left side, and no provision for the supply 

 of the upper extremity on the right side. It is as though the 

 vascular and the organopoietic layers had been shifted upon one 

 another. Nature provides a remedy by guiding the supernumerary 

 vessel behind the oesophagus and the trachea to the right upper 

 limb, thus placing the origin of the right subclavian below that of 

 the left one. 



Finally, to this early 23criod of development I must also refer 

 the congenital contraction or occlusion of the descending aorta. 

 At the point where the ligamentujn Botalli is attached to the 

 descending aorta, we find a sharply-defined contraction of the 

 entire tube. In the majority of the hitherto published cases, 

 the lumen of the vessel was entirely obliterated. A collateral 

 circulation had been established between the ascending and the 

 descending aorta by means of anastomoses between the first 

 intercostal, internal mammary, dorsalis scapuloe, subscapular, 

 thoracic and epigastric arteries. It is my belief that this oblitera- 

 tion, which liokitansld particularises as obliteration of the isthmus 

 aortce, occurs before the separation of the heart into a right 

 and a left half — at a time therefore when the blood can still pass 

 throuo;h the ductus arteriosus Botalli hito the descendino: aorta 

 (fig. 94) ; for a contraction of the aorta in opposition to the 

 high pressure of the blood would be contrary to all our experi- 

 ence of the mechanics of aneurism. 



