386 VARIETIES OF THE AORTA. 



artery may be entirely united in one simple stem in connection with a simple heart similar 

 to that of fishes. 



4. The Varieties in the Aortic Arch itself are intimately connected with the mode of 

 development of the fourth arterial arches. The natural aortic arch of man, and of all 

 mammalia, is a left one produced by the persistence and development of the left fourth 

 arch ; in birds it is the right fourth arch which forms the permanent aorta ; and in reptiles 

 both the right and left fourth arches remain patent. 



Reference may here be made to the complete lateral transposition which is occasionally 

 seen in the aortic arch and pulmonary arteries (fig. 333, D), as well as in the great veins and 

 the several divisions of the heart, and which may affect only these parts (dextrocardia), or 

 may be accompanied by a similar transposition of the viscera of the body generally {situs 

 inversus). Such cases are usually unattended by any disturbance of function or other 

 unnatural condition of structure. There is in fact only a change of position which may be 

 compared to that in which the natural parts would appear if viewed by reflection from a 

 mirror. From the direction of the apex of the heart towards the right, and other deviations 

 from the natural position, the existence of this transposition is capable of being ascertained 

 during life. 



The aortic arch has been observed completely double in some rare cases, which may be 

 explained on the supposition that both the right and left fourth arches have remained 

 pervious and undergone development. The ascending aorta, having the usual relation to the 

 pulmonary artery, divides above into two branches which pass backwards, embracing the 

 trachea and oesophagus, and join on the left side of the spine to form the descending aorta. 

 Each arch gives origin to the common carotid and subclavian arteries of its own side 

 (fig. 333, B). Examples of this condition have been recorded by Hommel, Curnow, and 

 some others. In one remarkable instance, however, known as Malacarne's case, the arrange- 

 ment was different, and seems to have been the result of some unusual mode of forma- 

 tion of the arterial stem. The ascending aorta divided close to the heart, and the two 

 arches embraced the trunk of the pulmonary artery, as well as the trachea and oesophagus ; 

 and each gave rise successively to a subclavian, an external, and an internal carotid 

 artery, an arrangement which is inconsistent with the known modes of development of the 

 vascular arches. 



The existence of a right aortic arch, that is, one passing to the right of the trachea and 

 gullet, instead of the usual left arch, is easily explained on the supposition of the right 

 fourth 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 

 beyond what proceeds from the change of side taken by the aortic arch, leading to the 

 innominate or brachio-cephalic artery being a left one, and the succeeding vessels being the 

 right carotid and right subclavian (fig. 333, E) : the recurrent laryngeal nerve forms 

 its sling on the right side round the aortic arch, and on the left round the arch of the 

 subclavian artery. But in the majority of cases of right aortic arch which have been 

 recorded there has been a farther irregularity affecting the origin of the left subclavian 

 artery, which arose independently of the carotid from the dorsal extremity of the arch in the 

 manner described in the next group. 



6. Varieties of the posterior part of the arch and ductus arteriosus belong properly 

 to changes occurring in connection with that portion of the primitive dorsal aorta (or as it is 

 often called the posterior aortic roof) which intervenes between the termination of the fourth 

 arch and the spot where the two trunks unite to form the single descending aorta. 

 In the normal condition this part persists on the left side, but is obliterated on the right. 



The most frequent variety of this group is that of the subclavian artery (of the right side 

 when the aortic arch is left or normal) arising from the back part of the arch, or fourth in 

 the series of vessels proceeding therefrom (fig. 334), a condition in which the subclavian 

 artery is continued from the primitive dorsal aorta (fig. 333. C), while the natural channel 

 through the fourth arch is closed. In such cases the subclavian artery takes its course 

 behind the trachea and gullet to reach its natural place between the scalene muscles and 

 over the first rib, and the inferior laryngeal nerve is not recurrent, but passes directly to the 

 larynx without being drawn down as a loop by the subclavian artery. 



A similar variety may occur with a right aortic arch, and the left subclavian artery is 

 then found to arise from the dorsal extremity of the arch by means of a kind of pouch which 

 represents the abnormally persistent part of the primitive dorsal aorta, and which receives 

 the insertion of the ligamentum arteriosum (fig. 333, F). The left recurrent laryngeal 

 nerve forms its loop below the arch thus constituted by the ligamentum arteriosum and the 

 beginning of the subclavian artery. There are also many instances of transitions or grada- 

 tions between these cases and the completely double aortic arch. 



In many of the cases of aberrant subclavian artery of the kind now referred to, the 

 vertebral artery is detached from the subclavian, and arises from the aorta in common with 

 the right carotid artery. Here the subclavian is derived from the dorsal aorta, while the 



