THORACIC CAVITY .115 



oesophagus (Fig. 57). The left side of the arch, which is 

 curved so that it is directed forwards as well as to the left, is 

 related to the mediastinal surface of the left pleura and lung, 

 but intervening between the pleura and the arch are (i)the 

 remains of the thymus gland, (2) the left phrenic nerve, 



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



(4) the superior cervical cardiac branch of the left sym- 

 pathetic, (5) the left vagus, and (6) the left superior inter- 

 costal vein. The vein passes upwards and forwards, lying 

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

 the right of the phrenic nerve (Figs. 47, 57). 



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 semi- 

 lunar cusps which are much stronger than the semilunar cusps 

 of the pulmonary valve (p. 102), 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 rule, lie 

 immediately above the level of the upper margins of the semi- 

 lunar 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 the 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. 58). 



Dissection. Divide the phrenic nerves immediately above 

 the diaphragm ; then, with the handle and the edge of the 

 scalpel, detach the lower part of the pericardium from the 

 diaphragm. The attachment of the pericardium to the muscular 

 part of the diaphragm is not close, and can easily be broken 

 down. The attachment to the central tendon is much more 

 firm and, as the median plane is approached, the aid of the edge 



