726 



HUMAN ANATOMY. 



cases the vertebral may arise as in the normal arrangement from the subclavian, or it maj-, as it 

 were, exchange positions with the subclavian, arising from the descending aorta, while the sub- 

 clavian arises, in common with the right common carotid, from an innominate stem ; or the 

 vertebral may arise with the right common carotid from the innominate stem, the subclavian 

 alone coming from the descending aorta (Fig. 686) . 



Group III. — A third group of anomalies depends upon the complete persistence of the 



right aortic arch, associated with the disappearance of the 

 Fig. 687. distal portion of the left one (Fig. 687). In such cases the 



result is a complete reversal of the aortic arch and its 

 branches, unaccompanied, however, by a reversal of any of 

 the other organs of the body, and thus differing from a 

 true situs inversus viscerum. The arch is directed from left 

 to right, and gives rise to an innominate stem, from which 

 the left common carotid and left subclavian arteries arise, a 

 right common carotid and a right subclavian, the descend- 

 ing aorta lying upon the right side of the vertebral column. 

 ^^^ Variations of these anomalies concern principally the rela- 



RAA J^r^.^Afe-~«L tions of the ductus arteriosus or the cord which represents 



DA — 1^— r-SH-^PIk:  ^t- ^^ ^^^y unite with the descending aorta, in which case 



it is the persistent right sixth branchial vessel, or it may be 



BS^\ p^J^^^i^-^^ T^S.Ui formed, as usual, from the left sixth branchial vessel, com- 



7" municating distally with the left subclavian, this artery, in 



cases where the ductus remains patent, appearing to arise by 



two roots, one from the innominate stem and one from the 



// pulmonary aorta. 



Group IV. — In the fourth group there is a complete 

 persistence of the right aortic arch associated with a dis- 

 Developmental variations appearance of the proximal portion of the left arch (Fig. 688), 

 of Group III. ^, aorta; P, the resulting arrangement being the reverse of that seen 

 S^Iorti arch-^''z?/duf i" cases belonging to the second group. The left sub- 

 tus arteriosus ; /?5, /:5,'right clavian arter>' appears to arise from the descending aorta, 

 and left subclavian arteries. which lies upon the right side of the vertebral column, and 



passes to the left behind the trachea and oesophagus. Varia- 

 tions in the relations of the ductus arteriosus, similar to those mentioned as occurring in the 

 third group, may be found. 



Group V. — A fifth group includes those cases in which Fig. 688, 

 the arch itself is normal, but in which there are xariations m the 

 vessels that arise from it. These variations may be either a 

 diminution or an increase of the normal number of vessels or 

 an abnormal arrangement of a normal number. The diminu- 

 tion and altered arrangement of the vessels depend upon a 

 shifting of more or fewer of them, so that, for example, the left 

 common carotid and left subclavian arteries may arise from a 

 common left innominate stem, all the vessels may arise from 

 a common stem, the two common carotids may have a com- 

 mon origin, while the two subclavians arise independently, or, 

 what is the most frequent of these variations, the left com- 

 mon carotid may arise from the innominate stem and pass j.„ | 



upward and to the left obliquely across the front of the ^^^ ^ i^\„ ^^^is 

 trachea. ^^ 



An increase in the number of vessels may be brought 

 about by the independent origin from the arch of both the 

 right common carotid and the right subcla\ian, the innominate 

 being absent. In other cases, vessels which normally do not 

 come into relation with the arch may take origin from it, this 



being most frequently the case with the vertebral arteries and Developmental variations 



less frequently with the internal mammaries ; and, finallj', an of Group iv. ^, aorta; .P, 

 additonal branch to the thyroid gland, the art. thyroidea ima, Pijht°' and ''[eft'^' subdavi^ 

 occasionally takes origin from the arch. arteries. 



Practical Considerations. — The Aortic Arch and Thoracic Aorta. — 



S7(rface Relations. — The ascending- aorta begins beneath the sternum just to the 

 right of the inner end of the third left costal cartilage. It ascends obliquely and 

 towards the upper border of the second right costal cartilage. The second (trans- 

 verse) part passes backward and to the left, crossing the mid-line about an inch 

 from the suprasternal notch, the lower ('concave) border corresponding in level with 

 the ridge betw^een the manubrium and the gladiolus, the upper (convex) border to the 

 level of the third thoracic spinous process, to the middle of the manubrium, and the 

 middle of the first costal cartilage. This border is about one inch below the supra- 

 sternal notch. The surface relations of this portion vary with the development of 



