AORTIC ARCH AND BRANCHES. 327 



portion of the aorta, it is divided into three parts ascending, transverse, 

 and descending. 



The first or ascending part is about two inches in length, or slightly 

 more, and is directed upwards behind, and close to the sternum : it reaches 

 as high as the upper border of the cartilage of the second rib on the right 

 side, and is contained nearly altogether in the pericardium. At first the 

 pulmonary artery is superficial to it ; but, as the vessels take different 

 directions, the aorta is soon uncovered, and remains so to its termination. 

 Behind it are the right branches of the pulmonary vessels. On the right 

 side is the descending cava ; and on the left, the pulmonary artery. Near 

 the heart the vessel bulges opposite the semilunar valves (fig. 105). There 

 is sometimes another dilatation along the right side, which is named the 

 great sinus of the aorta. 



The second or transverse portion recedes from the sternum and reaches 

 from the second right costal cartilage to the left side of the body of the 

 fourth dorsal vertebra (the lower border). It rests upon the trachea above 

 the bifurcation, and is placed over the oesophagus and the thoracic duct. 

 Lying in front of this part of the artery are the vagus, phrenic, and super- 

 ficial cardiac nerves of the left side the first nerve sending backwards its 

 recurrrent branch beneatli the vessel. Along the upper border is the left 

 innominate vein (<?), to which the left upper intercostal vein (Y) is directed 

 over the left part of the arch ; and to the lower border near the termina- 

 tion, the remnant of the arterial duct (e) is attached. From this part 

 arise the three great vessels of the head and upper limbs. 



The third or descending part of -the arch is very short, extending from 

 the lower edge of the fourth to that of the fifth dorsal vertebra. 1 It lies 

 against the fifth vertebra, and the fibro-cartilage between this and the 

 fourth, and is covered by the pleura of the left side of the chest. 



In the concavity of the arch of the aorta are contained the root of the 

 left lung, the branching of the pulmonary artery with its arterial duct, and 

 the left recurrent nerve. Deeper than those parts, the oesophagus and the 

 thoracic duct, with some lymphatic glands, may be recognized. 



The branches of the arch of the aorta are five in number; two come 

 from the ascending, and three from the transverse part. The first two 

 are the coronary arteries of the heart (o), which have been already no- 

 ticed (p. 313). The other three are much larger in size, and supply the 

 neck, the head, and the upper limbs. First on the right is the large trunk 

 of the innominate artery (b) ; close to it is the left carotid (c) ; and last of 

 all comes the left subclavian (d). 



The INNOMINATE ARTERY (b~) (brachio-cephalic), the first and largest 

 of the three branches, measures from one inch and a half to two inches in 

 length. Ascending to the right beneath the sternum, it divides opposite 

 the sterno-clavicular articulation into the right common carotid and the 

 subclavian artery. 



The artery is crossed by the left innominate vein (</), and lies behind 

 the upper piece of the sternum, and the origin of the hyoid and thyroid 

 muscles. At first it rests on the trachea, but as it ascends it is placed on 



1 It is usually said that the second part of the aortic arch ends opposite the left 

 side of the body of the second dorsal vertebra, and the third part, opposite the 

 lower edge of the third vertebra. After examination, I have adopted the state- 

 ment of Mr. Wood respecting the position of the aortic arch to the dorsal vertebrae. 

 (Journal of Anatomy and Physiology for 1868.) 



