724 



HUMAN ANATOMY. 



The transverse portion is crossed on its anterior surface by the left phrenic, 

 cardiac, and pneumogastric nerves, arranged in that order from right to left, the 

 pneumogastric crossing it on a level with the origin of the left subclavian artery. 



Remains of th\mus 

 II. costal cartilage \ 



Fig. 680. 



Sternum 



.Pericardial sac 



Lung 



l«ft phrenic nerve 



Ascending portion 

 Bronchial lymph-node 

 Pleural sac 



Transverse portion 



Left pneumogastric nerve 



Left recurrent laryngeal nerve 



Descending portion 



Lung 



Right phrenic nerve 

 Superior vena cava 



Bronchial lymph-node 



CEsophagus 

 Thoracic duct 



— Bifurcation of trachea 

 ^fe| — Right pneumogastric nerve 



- Vena azygos 



IV. thoracic vertebra 



Fig. 681. 



Fig. 682. 



Part of cross-section of body at level of fourth thoracic vertebra, viewed from above; 

 upper part of aortic arch has been removed. 



More posteriorly the anterior surface is in contact with the left pleura. Behind it is 

 in relation from right to left with the superior vena cava, the trachea, the oesoph- 

 agus, and the body of the fourth thoracic^ertebra, and below it are the right pul- 

 monary artery, the left recurrent laryngeal nerve, and the left bronchus, the arch 

 crossing this last structure obliquely from above downward and outward. 



The descending portion of the arch has in front of it a portion of the left 

 pleura and the root of the left lung. Behind, it rests upon the fourth thoracic ver- 

 tebra; to the right of it are the 



oesophagus and the thoracic duct 



and also the body of the fourth 



thoracic vertebra, and to the left 



are the left pleura and lung. 



Branches. — Just above its 



origin the aortic arch gives off 



( I ) the right and left coronary 



arteries, and from the upper or 



convex surface of the transverse 



portion there arise in succession, 



from right to left, (2) s\-\^ innom- 

 inate or brachio-ccphalic, (3) the 



left commoyi carotid, and (4) the 



left subclavian artery. 



Variations.— Owing to the com- 

 plexity of the changes by which the 

 primary arrangement of the branchial 

 arch vessels is transformed into the 

 adult arrangement (Figs. 681, 682), 

 and owing also to the possibility of 

 some of the normal changes remaining 

 uncompleted, the variations which oc- 

 cur in connection with the arch of the 

 aorta are rather numerous. They may 

 be convenientiv classed in five groups. 

 Group I.— In the normal development (Fig. 682) the distal portion of the right aortic arch 

 degenerates as far up as the right subclavian artery, indications of it persisting as a more or 

 less rudimentary vas aberrans arising from the thoracic aorta. This degeneration may not 

 occur, both the right and left aortic arches persisting in their entirety (Fig. 683); and, since in 



Diagram showing primary 

 arrangement of longitudinal 

 stems and series of six aortic 

 bows; 7>1, truncus arteriosus ; 

 VA, DA, ventral and dorsai 

 aorta;; A, unpaired dorsal 

 aorta; I-VI, aortic bows, of 

 which ^^ is rudimentary. 



Diagram showing normal 

 derivations in man of primary 

 vessels by modification of pre- 

 ceding plan; y4, aorta; ^.^, aortic 

 arch; /, innominate artery ; CC, 

 common carotids; EC. IC, exter- 

 nal and internal carotids ; 6", sub- 

 clavian artery ; P, pulmonary art- 

 ery; /?^, ductus arteriosus. 



