THORACIC CAVITY 85 



to the left lie the left auricle below, and the upper part of the 

 stem of the pulmonary artery above. 



Branches. Only two branches are given off from the 

 ascending part of the aorta; they are the right and left 

 coronary arteries. The right springs from the anterior 

 aortic sinus and the left from the left posterior sinus. Their 

 distribution has been described already (p. 60). 



The Superficial Cardiac Plexus. Before the arch of the 

 aorta is studied, the position, connections and relations of the 

 superficial cardiac plexus should be defined. It lies below 

 the arch of the aorta, above the bifurcation of the stem of the 

 pulmonary artery, and between the ascending aorta on the 

 right and anteriorly, and the ligamentum arteriosum to the left 

 and posteriorly. The positions of the superior cervical cardiac 

 branch of the left sympathetic trunk, and the inferior cervical 

 cardiac branch of the left vagus, on the left side of the 

 arch of the aorta, have been defined already (p. 33). 

 Trace these nerves to the plexus, clear away the areolar 

 tissue from around the plexus, and trace branches posteriorly 

 and upwards from it towards the deep cardiac plexus, which 

 lies posterior to the arch of the aorta. Other branches which 

 spring from the superficial part of the cardiac plexus descend 

 along the pulmonary artery and form the right coronary 

 plexus, which is distributed with the right coronary artery. 



The Arch of the Aorta. The aortic arch commences at the 

 termination of the ascending part of the aorta, at the level of 

 the second costal cartilage, and posterior to the right margin of 

 the sternum, from which it is separated by the anterior part 

 of the mediastinal portion of the right pleura and lung, or 

 by the remains of the thymus gland (see Fig. 43). It runs 

 posteriorly, to the left, and slightly upwards, through the middle 

 mediastinum and round the left margins of the trachea and 

 oesophagus (see Figs. 13 and 43), to the level of the lower 

 border of the left side of the fourth thoracic vertebra, where it 

 becomes continuous with the descending part of the aorta. It 

 is curved in both the vertical and the horizontal planes, and as 

 it passes posteriorly and to the left it forms a convexity 

 upwards, and also a convexity which is directed anteriorly and 

 to the left. Its lower border is connected with the left 

 pulmonary artery by the ligamentum arteriosum, and from its 

 upper border arise the three great vessels which supply the 

 head, neck, and upper extremities. 



