352 



THE AORTA. 



which cannot be explained by what is as yet kncmi of the modes of develop- 

 ment of the branchial vascular arches. 



The existence of a rhiht aortic arch, that is, one passing' to the right of the 

 trachea and giillet, instead of the usual left arch, is easily explained on the sup- 

 position of the fourth rig-ht branchial arch having been developed instead of the 

 left ; and accordingly there are instances of this variety, in which no other 

 deviation from the natural condition of the parts exists, l^eyond what proceeds 

 from the change of side taken bj- the aortic arch, leading to the innominate or 

 brachio-cephalic artery being a left one, or furnishing the left subclavian and 

 carotid arteries, and the succeeding vessels being the right carotid and right sub- 

 clavian. The recuiTent larjTigeal nerve forms its sling on the right side, round 

 the aortic arch , and on the left round the arch of the subclavian artery. 



Under the same division as the foregoing might also be brought those remark- 

 able and numerous cases of varieties in the mode of closure of the ductus arteri- 

 osus and of its union with the aortic root or other vessels ; but these and other 

 modifications of the variations of the arches may best fall iinder the next 

 division. 



4. Varieties of the [wsterior j'art of the nrch and ductus artcriofius belong pro- 

 perly to the changes occun-ing in connection with the posterior emljryonic aortic 

 roots. 



One of the most frequent varieties of this group is that of the subclavian artery 

 (of the right side, when the aortic arch is left or normal) rising, as it has been 

 described, from the back part of the arch, or fourth in the series of vessels pro- 

 ceeding from it, but which, according to embryological elucidation, would be more 

 correctly designated as the subclavian artery formed in connection with one of 

 the posterior aortic roots ; the natural anterior root and arch being abnormally 

 closed. In such cases the subclavian artery takes its coui'se behind the trachea 

 and gullet to reach its subsequent natural place as it i^asses between the scalene 

 muscles and over the first rib. 



Fis. 24i 



Fig. 245. — Diagram of the natural origin of Vessels from 

 THE Aortic Arsh as compared with the Displaced Sub- 

 clavian Arterv. 



(I). The normal disposition ; (II), tlie right siibclavian artery 

 displaced or proceeding from the right aortic root. A, A, 

 ascending and descending jiarts of the tlioracic aorta ; P, pul- 

 monary stem ; d, ductus arteriosus ; a, right aortic root or its 

 remains ; a', left aortic root ; r, common carotid arteries , ■/, 

 innominate artery ; s, right, and s', left subclavian arteries ; Vy. 

 right, and v', left vertebral arteries. 



A similar variety may occur in an instance of right 

 aortic arch, in which case it is the left subclavian artery 

 which takes the abnormal course. In both examples of 

 this variety, the anterior part of the subclavian arch being 

 absent, the inferior laryngeal nerve is not recmrent, but 

 passes directly to the larynx, without being drawn do'mi 

 as a sling or looji by the subclavian artery. 



There are also many instances of transitions or grada- 

 tions between these cases and the completely double aortic 

 arch. See a paper on " Varieties of the Arch of the 

 Aorta," by Turner, in Brit, and For. Med. Chir. Eeview, 

 18G3, and '• Description of a Case of Right Aortic Arch." 

 by Allen Thomson, in Glasg. Med. Journ. 1862. in which 

 such cases are more fully explained, and also the works 

 of Kenle, Hyrtl and Stiuthers. 



In some of the instances of aberrant subclavian artery 

 of the kind now referred to, the vei-tebral artery is 



detached from the subclavian, and rises directly from the arch of the aorta. 

 Under the same division may be brought those numerous varieties in the 



closure of the ductus arteriosus and its union with the aorta or other vessels. 



