338 



THE AORTIC ARCH BRANCHES. 



remain pervious : in a case of right aortic arch, the fourth arch of the right side, 

 instead of that of the left, has remained pervious and has taken on permanent deve- 

 lopment, while at the same time adjusting deviations from the usual process of 

 development have occurred in parts of the other arches, which lead to a left in- 

 nominate being the first great vessel rising from the arch, and to the right carotid 

 and right subclavian arteries rising later in succession. Thus, too, transference of 

 the right subclavian artery from its usual connection with the innominate of a natural 

 left aortic arch to the fourth place of origin, and its remarkable passage behind the 

 trachea and gullet, may probably proceed from the obstruction of the part of the 

 fourth right arch which unites the aortic bulb with the aortic root, and is accompanied 

 by persistence of the aortic root itself extending from the main aorta below upwards 



Fig. 252. 



Fig. 252. DIAGRAMS TO ILLUSTRATE THE 



RELATION OF SOME ABNORMAL DISPOSI- 

 TIONS OF THE AORTIC ARCH AND ITS 

 BRANCHES TO THE NORMAL CONDITION. 



(I). The normal disposition as illus- 

 trated by Fig. 245, p. 326 ; (II), an 

 abnormal right aortic arch ; (III), a left 

 aortic arch with the right subclavian artery 

 displaced to the right aortic root ; (IV), 

 an abnormal right aortic arch with the 

 left subclavian displaced to the left aortic 

 root. Upper A, ascending part of the 

 aortic arch ; lower A, descending thoracic 

 aorta ; P, pulmonary trunk ; d, ductus 

 arteriosus ; a, right aortic root or its re- 

 mains ; a', left aortic root ; c, common 

 carotid arteries ; i, innominate artery ; 

 s, right, and s', left subclavian arte- 

 ries ; v, right, and v' t left vertebral 

 arteries. 



to the subclavian artery. The similar 

 transference of a left subclavian artery, 

 in combination with the anomaly of 

 a right aortic arch, may be due to similar 

 abnormal states of development occur- 

 ring on a different side. So, also, many 

 of the other less marked variations in 

 the number and position of the perma- 

 nent branches proceeding from the aortic arch probably owe their origin to cognate 

 departures from the usual process of change in those parts of the original vascular 

 arches with which their roots are connected. 



By reference to development we are likewise enabled to understand how the right 

 aortic arch of the bird, and the double aortic arch of reptiles, arise by obliteration 

 or permanence of different members of a series of branchial arches comparable with 

 those of fishes. 



(Full reference to the history of cases of aortic varieties is given by Turner, " On 

 Varieties of the Arch of the Aorta," Brit. & For. Med. Cher. Review, 1863 ; and an 

 account of the origin of the varieties as explained by the observation of the develop- 

 ment of the vessels is given in the same paper, and in that of A. Thomson, 

 " Description of a Case of Right Aortic Arch," &c., Glasgow Med. Journ. 1862.) 



BRANCHES OF THE ARCH OF THE AORTA. 



THE CORONARY ARTERIES. 



The coronary or cardiac arteries are two small vessels, named right and 

 left, which arise from the root of the aorta in the upper parts of the two 



