THE AORTIC ARCH. 



725 



such cases the descending aorta usually retains its normal position to the left of the spinal 

 column, a condition is produced in which the aortic arch appears to be split lengthwise into 

 two portions, one of which, the left arch, passes in front of the trachea and oesophagus and 

 gives origin to the left common carotid and the left subclavian arteries, while the other passes 



Fig. 683. 



Fig. 684. 



SAA 



Right common carotid  



LAA 



i 



Developmental variations of 

 Group I, giving rise to anomaly 

 shown in next figure. RAA, LAA, 

 right and left aortic arches; RS, 

 LS, subclavian arteries ; A, aorta ; 

 P, pulmonary artery. 



Pulmonary artery 



Double aortic arch through which trachea 

 and oesophagus pass. (Hommel) . 



behind the structures named and gives origin to a right common carotid and a right subclavian 

 (Fig. 684). 



The relative diameters of the two portions of the aortic arch so formed may vary con- 

 siderably, that passing in front of the trachea (the true left arch) being sometimes larger and at 

 other times smaller than the other one. In the latter case an obliteration of the distal portion of 

 the left arch may occur, and the left common carotid and left subclavian arteries will then 

 appear to arise close to the innominate stem, from a common trunk, the aortic arch passing 

 to the left behind the trachea. 



Group II. — A more frequent anomaly is the complete persistence of the distal portion of 

 the right aortic arch (Fig. 685) associated with the disappearance of a greater or less portion of 



Fig. 685. 



Fig. 686. 



JiV 



Trachea 



Right common 

 carotid 

 Right 

 vertebral 



Right 

 subclavian 



R8 



Developmental variations oi 

 Group II, giving rise to anomaly 

 shown in next figure. A, aorta; 

 P, pulmonary artery ; RS, LS, right 

 and left subclavian arteries; Ry, 

 right vertebral artery. 



Aorta 



.CEsophagus 



Left common carotid 



Left vertebral 



Left 

 subclavian 



Right 



subclavian 



Origin of right subclavian artery from descending aorta. 



its proximal part, the result being the apparent origin of the right subclavian artery from the 

 descending aorta, whence it passes to the right behind the trachea and oesophagus. Variations of 

 this condition, depending upon the location and extent of the disappearing portion of the right 

 arch, may modify the relations of the right vertebral and subclavian arteries. Thus, in some 



