384 THE ARCH OF THE AOKTA. 



extent on the left, by the pleura and lung. It is crossed on the left side by the left 

 pneumo-gastric and phrenic, and the superficial cardiac nerves, as well as more 

 posteriorly by the left superior intercostal vein ; and the recurrent laryngeal branch 

 of the pneumo-gastric turns upwards beneath it. Behind and to its right side are 

 placed the trachea with the deep cardiac plexus, the left recurrent laryngeal nerve, 

 the oesophagus and thoracic duct, and the body of the fourth dorsal vertebra. The 

 upper border of the arch has in contact with it along its anterior half the left inno- 

 minate vein ; and from it are given off the large arteries (innominate, left carotid 

 and left subclavian) which are furnished to the head and upper limbs. The lower 

 or concave border overhangs the left bronchus and the bifurcation of the pulmonary 

 artery, and is connected with the left branch of that artery by the ligamentous 

 remains of the ductus arteriosus, which is attached to the aorta just beyond the place 

 at which the left subclavian artery springs from its upper aspect (fig. 312). In the 

 interval between the arch and the bifurcation of the pulmonary artery are lodged the 

 superficial cardiac plexus and several large bronchial lymphatic glands ; and along 

 its lower and fore part the fibrous pericardium becomes united with its external coat. 



The arch of the aorta is considerably reduced in size (to 23 mm.) after having 

 given off the large branches, and it often presents, beyond the origin of the sub- 

 clavian artery, at the part corresponding to the place of attachment of the ligament 

 of the ductus arteriosus, a marked constriction known as the aortic isthmus, 

 which is succeeded by a fusiform dilatation the aortic spindle of His, extending to 

 the beginning of the descending thoracic aorta. 1 In the foetus the isthmus is more 

 distinct, the entrance of the ductus arteriosus causing a sudden expansion of the 

 contracted aortic trunk. 



BRANCHES. From the arch of the aorta are given off the three large primitive 

 trunks, which supply the head and neck, the upper limbs, and, in part, the thorax. 

 They arise from the upper aspect of the arch, in the following order : first, the 

 innominate or brachio-cephalic artery ; second, the left common carotid ; and 

 third, the left subclavian artery. The origin of the left carotid artery is usually 

 nearer to the innominate artery than it is to the subclavian artery of its own side. 



Varieties of the aorta, &c. It will be convenient to consider together in this place the 

 varieties of the aorta generally, those of the pulmonary artery, and irregularities in the 

 arrangement of the branches given off from the aortic arch. The conditions to be referred to 

 are for the most part the result of abnormal modes of development affecting the primitive 

 arterial trunks and arches of the embryo, such as irregular position or defective development 

 of the aortic septum, the persistence of channels that are usually obliterated, with more or less 

 extensive occlusion of the normal passages, and variations in the degree of fusion of adjacent 

 vessels ; and the manner in which some of the more important of these, as well as the 

 normal arrangement, are related to the embryonic condition is shown in the diagrams in 

 fig. 333. For a full account of the development of these arteries in the foetus the reader is 

 referred to the section on Embryology in Volume I. 



1. The aorta may vary in its position and extent, without other irregularity in course or 

 relations. Thus, the height to which the arch rises in the upper part of the chest is found 

 to be subject to variation to the extent of one or two vertebras ; so that while in some 

 instances the summit of the arch is on a level with the third dorsal vertebra, reaching as 

 high as the top of the sternum, in other cases it is as low as the fifth dorsal vertebra. 



The distance to which the aorta extends downwards depends on the seat of its division 

 into the common iliac trunks, which frequently varies to the extent of a lumbar vertebra, 

 so that the place of division may be as low down as the fifth or as high up as the third. In 

 rare cases the division occurs still higher. 



The position of the aorta with reference to the median plane or vertebral column is also 

 subject to some variation, but such deviation to the side is more frequently the result of 

 pathological changes than of congenital malformation. 



1 W. His, "Anatomie menschlicher Embryonen," iii. 196 ; H. Stahel, " Ueb. Arterienspindeln 

 u. ub. d. Beziehung d. "Wanddicke d. Arterien z. Blutdrucke," Arch. f. Anat., 1886. 



