VARIATIOyS IN ARCH OF AORTA 



All 



(1) Variations i)i the Aortic Arch itself 



The variations in the aortic arch itself may for the most part be traced to abnormahties in 

 development of the embryunic stnictures from which the three portions of the arch and the pul- 

 HKinary arteries and ductus arteriosus are derived, i.e. («) the jirimitive ventral aortic stem ; 

 {b) the i'uurth and fifth branchial arches ; and (c) the dorsal aortic stems. 



(«) Variations depending on Ahnormah'tits in Development of the Ventral 



Aortic Stem 



The.^e variations are closely associated with abnormalities of the pulmonary artery and heart. 

 They depend upon deficient development of the septum which normally divides the anterior 

 stem into the first portion of the aorta and the pulmonary arterj'. The following are some of 

 the chief variations : — 



(1) The aorta and ]uilmonary artery may ari.se as a single stem fi-om a simple heart. The 

 septum here is completely absent. The condition resembles the normal state in fishes. 



(2) The aorta and pulmonary artery may be more or less conjoined, and the septum of the 

 heart incomplete. This is analo.trous to the normal condition in reptiles. 



(3) The aorta and pulmonary artery may be transposed. 



(4) The aorta or pulmonary artery may be in part obliterated, and the blood earned into the 

 remainder of the affected vessel through an abnormal opening beyond the obliterated part. 



Fig. 325. — Scheme of the Relations of the Third Poetiox of the 

 Akch of the Aorta. (Walsbam.) 



THORACIC DUCT 



LEFT BRONCHUS 



LEFT LUNG AND PLEURA 



Descending pail of arch of 

 aorta 



Left pulmonary artery 



{h) Variations dependinf/ on Abnormalities in Development of the Fonrth and 

 Fifth Aortic or Branchial Arches 



(1) The aorta may be double, the normal condition in amphibians, each arch giving ofi" a 

 corresponding carotid and sulx-lavian artery. This abnormality is explained by the persistence of 

 the right fourth aortic arch and right descending aortic stem, as well as the con-esponding parts 

 on the left side. The trachea and oesophagus pass through the arterial circle thus formed. 



(2) The aortic arch may turn over the right bronchus instead of over the left. This is the 

 normal condition in birds, and is exi)lained by the jiersistence of the right fourth aortic arch and 

 right descending aortic stem and the obliteration of the corresponding parts on the left side. 



(3) The puimonaiy artery may be given off froTn the aorta. This condition is due to the 

 obliteration of the anterior part of the fifth branchial arch and the persistence of the posterior 

 part which normally constitutes the ductus arteriosus. 



(c) Variations depending on Abnormalities in Development of the Dorsal Aortic Stems 



The right subclavian may arise from the third part of the arch of the aorta. It then j.asses be- 

 hind the trachea and oesophagus, and in front of the vertebral column, to gain the interval between 

 the right scalene muscle.*. The explanation of this abnormality is that the right fourth arch, 

 from which the right subclavian is normally developed, is obliterated, whilst the right jfosterior 

 aortic stem remains pervious as far as the spot where it normally joins the right fourth arch, the 

 blood thus passing through the aortic stem instead of the arch. The recurrent laryngeal nerAe in 

 these cases runs .straight to the larynx, since, the fourth arch being obliterated, the nerve is no 

 longer hooked down by it. Rudiments of the right aortic stem fretiuently persist as the aberrans 

 branch of the superior intercostal artery and thoracic aorta. 



