96o A MANUAL OF ANATOMY 



ficial cardiac plexus, which they form. All these nerves are over- 

 laid by the left pleura. Farther back than these nerves is the 

 left superior intercostal vein. The remains of the . thymus body 

 constitute an additional anterior relation. Posterior. — ^The trachea, 

 deep cardiac plexus of nerves, oesophagus, thoracic duct, and left 

 recurrent laryngeal nerve. Superior. — The left innominate vein, 

 and the origins of the following three great arteries, named in 

 order from right to left, and also from before backwards : the 

 innominate, the left common carotid, and the left subclavian. 

 Inferior. — The left bronchus, the bifurcation of the pulmonary 

 artery, the superficial cardiac plexus of nerves, the left recurrent 

 laryngeal nerve, and the ligamentum ductus arteriosi. The last- 

 named fibrous cord is attached to the back part of the concavity 

 of the arch, immediately beyond the level of the origin of the left 

 subclavian artery. 



The arch presents a constriction immediately beyond the origin 

 of the left subclavian artery, called the aortic isthmus, and this is 

 succeeded by a short fusiform dilatation, known as the aortic 

 spindle of His. These conditions are best marked in the foetus. 



Branches. — ^These are three in number, namely, the innominate, 

 left common carotid, and left subclavian arteries. They arise in 

 the order named, proceeding from before backwards, and also 

 from right to left. 



Innominate Artery. — ^This vessel is the first and largest of the 

 three branches which arise from the arch of the aorta. It springs 

 from the upper aspect of the arch rather above the level of the 

 upper border of the second right costal cartilage, and it terminates 

 behind the upper border of the right sterno-clavicular joint by 

 dividing into the right common carotid and right subclavian arteries. 

 It is from i| to 2 inches in length, its direction is upwards and 

 outwards, and it lies in the superior mediastinum. 



Relations — Anterior. — ^The right half of the manubrium sterni, 

 ■ with the origins of the right sterno-hyoid and sterno-thyroid 

 muscles ; the left innominate and the right inferior thyroid veins ; 

 the right sterno-clavicular joint ; and some remains of the thymus 

 body. Posterior. — ^The trachea at first, but, as the artery ascends 

 obliquely to the right, it leaves the front of the trachea and is 

 placed on its right side. The three cervical cardiac branches of 

 the right sjnnpathetic also lie behind the artery on their way to 

 the deep cardiac plexus. Right. — ^The right pleura ; the innominate 

 vein, with the right pneumogastric nerve behind it ; the sui:)erior 

 .vena cava ; and the right phrenic nerve. Left. — ^The left common 

 carotid artery, and the trachea, in this order from below upwards. 



Branches. — These are terminal, and are two in number, namely, 

 the right common carotid and the right subclavian. The artery, 

 as a rule, gives off no branches in its course. Occasionally, how- 

 ever, it gives origin to a vessel, of variable size, called the arteria 

 ihyroidea ima (lowest thyroid artery). The interest attached to 

 this occasional branch is that, in ascending to the isthmus of the 



