THORACIC CAVITY 87 



mentum arteriosum, which connects the left pulmonary artery 

 with the arch ; (3) the superficial part of the cardiac plexus 

 immediately to the right of the ligamentum arteriosum ; (4) 

 the left recurrent nerve on the left side of the ligament ; and (5) 

 still further to the left, the left bronchus passes beneath the 

 arch on its way to the hilus of the left lung. To the right of the 

 arch are the trachea, the oesophagus, the left recurrent nerve, 

 and the thoracic duct. The nerve lies in the angle between 

 the oesophagus and the trachea, and the thoracic duct is pos- 

 terior to and to the left of the oesophagus (Fig. 43). The left 

 side of the arch is overlapped by the posterior part of the media- 

 stinal surface of the left pleura and lung, but intervening 

 between the pleura and the arch are (i) the left phrenic nerve, 

 (2) the inferior cervical cardiac branch of the left vagus, (3) 

 the superior cervical cardiac branch of the left sympathetic, 

 (4) the left vagus, and (5) the left superior intercostal vein. 

 The vein passes upwards and anteriorly, lying to the left of 

 the vagus and the cardiac nerves, and to the right of the 

 phrenic nerve (Fig. 13). 



Dissection. Divide the right coronary artery close to its origin. Cut 

 through the anterior wall of the ascending part of the aorta on each side of 

 the anterior aortic sinus ; extend the incisions upwards to the commencement 

 of the aortic arch, and examine the aortic valve. Note that it is formed by 

 three semilunar cusps which are much stronger than the semilunar cusps of 

 the pulmonary valve (p. 75), but are exactly similar in structure and 

 attachments. Note further that one cusp lies anteriorly, and the other two 

 posteriorly. Examine the aortic sinuses and note that the right coronary 

 artery springs from the anterior sinus, and the left coronary from the left 

 posterior sinus. Note further that the orifices of the coronary arteries, as a 

 rale, lie immediately above the level of the upper margins of the semilunar 

 cusps. Replace the stem of the pulmonary artery in position, and note the 

 relative positions of the pulmonary, aortic, and atrio-ventricular orifices. 



Topography of the Great Orifices of the Heart. Replace 

 the sternum in position and note the relations of the cardiac 

 orifices to that bone. The pulmonary orifice is highest. It 

 lies to the left of the margin of the sternum at the level of the 

 third costal cartilage. The aortic orifice is a little lower, and 

 more to the right, posterior to the left margin of the sternum, 

 at the level of the third left intercostal space. Below the aortic 

 orifice is the left atrio-ventricular orifice, posterior to the left 

 margin of the sternum at the level of the left fourth costal 

 cartilage. Still lower and more to the right is the right atrio- 

 ventricular orifice, posterior to the centre of the sternum at the 

 level of the fourth intercostal spaces (Fig. 44). 



